It seems customary to share merit in certain Buddhist traditions. However, I've recently heard merit can't really be shared. What did the Buddha say about this?
The Buddha said:
AN10.91:23.1: In the same way, the pleasure seeker who seeks wealth using legitimate, non-coercive means, who makes themselves happy and pleased, and shares it and makes merit, and who uses that wealth untied, uninfatuated, unattached, seeing the drawbacks, and understanding the escape is the foremost, best, chief, highest, and finest of the ten.”
With an unfettered heart, how could there be a distinction between merit and sharing?
The following passage is usually used as a support for sharing merit but i now think it may not be although i am not sure.
Master Gotama, you know that we brahmans give gifts, make offerings, [saying,] 'May this gift accrue to our dead relatives. May our dead relatives partake of this gift.' Now, Master Gotama, does that gift accrue to our dead relatives? Do our dead relatives partake of that gift?"
"In possible places, brahman, it accrues to them, but not in impossible places." "And which, Master Gotama, are the possible places? ... "Then there is the case where a certain person takes life, takes what is not given, engages in sensual misconduct, engages in false speech, engages in divisive speech, engages in abusive speech, engages in idle chatter, is covetous, bears ill will, and has wrong views. With the break-up of the body, after death, he reappears in the realms of the hungry shades. He lives there, he remains there, by means of whatever is the food of hungry shades. He lives there, he remains that, by means of whatever his friends or relatives give in dedication to him. This is the possible place for that gift to accrue to one staying there.
"But, Master Gotama, if that dead relative does not reappear in that possible place, who partakes of that gift?"
"Other dead relatives, brahman, who have reappeared in that possible place."
"But, Master Gotama, if that dead relative does not reappear in that possible place, and other dead relatives have not reappeared in that possible place, then who partakes of that gift?"
"It's impossible, brahman, it cannot be, that over this long time that possible place is devoid of one's dead relatives. An10.177
It has been pointed out that it may be about in some way attempting to have a dead relative receive a gift.
Afaik the transfer of merit to living people isn't canonical to EBTs and it would indeed be strange if one could experience the good consequences of another's work as it is generally held that no one can purify another.
The chants akin to Uddissanaadhitthaana Gaathaa do not appear in canonical texts, that one in particular is composed by king Rama 4th of Thailand.
The sharing of merits happens when you make your good deed to be known by others so that they can rejoice or give assistance to it.
Having been given in proper season,
with hearts inspired by the noble ones
their offering bears an abundance.
Those who rejoice in that gift
or give assistance,
they, too, have a share of the merit,
and the offering isn’t depleted by that.
So, with an unhesitant mind,
one should give where the gift bears great fruit.
Merit is what establishes
living beings in the next life.
It's good to mention that, according to AN 8:39, the abstention of doing bad deeds is also a way to "give a gift"
There are already other answers, good householder.
In addition to those, one can share merit by inviting others to join in the carrying out of charitable or meritorious activities, and increase right view, if wished. For e.g. when giving alms to monks, invite others to join you in giving alms, and thereby sharing in the merits.
It's of course very much possible that one gravely addicted to worldly sensual pleasures, would not want to share in joint charitable or meritorious activities.
Another way to share merits is to tell others about the charitable and meritorious activities that one has done, so that others may practise mudita (appreciation, approve, rejoice with). For e.g. when you have given alms to monks, tell others so that they may rejoice in your act and thereby share your merits by practicing mudita.
But mudita cannot be practised by those who do not have right view.
Some explanation here (switch language for english.) The other answer here, by the way, has nothing to do with the sharing of merits, but rather talks about being charitable to those who have passed on.
There is this explaination in Pv5
The Supreme Buddha gave this sermon to King Bimbisara after an alms offering to the Sangha.
After they have been born in the ghost world, departed relatives will come back to their own houses and stand by the doors. They also stand outside walls and at intersections.
Some people in the family will enjoy delicious food without remembering their departed relatives. Departed relatives are forgotten because of their own bad karma.
Some compassionate people offer delicious food and drink to virtuous people and share merits with their departed relatives saying, “Let this be for our relatives! May our relatives be happy!” Departed relatives gather to these places and highly appreciate the offering. They bless their relatives saying, “May our relatives who compassionately offered us these gifts have long, happy, and healthy lives.” The givers also gain good results.
Beings in the ghost world do not farm, herd cattle, trade, buy, sell, or use gold and money. They survive on merits shared by humans. As water that rains on a mountain-top flows down to the bottom, so will the merits shared from the human world reach the beings in the ghost world. Just as streams of water fill the ocean, so will the merits shared from the human world reach the beings in the ghost world. One should share merits with departed relatives recalling, “He gave to me, he worked for me, he was a relative, friend, and companion.”
Weeping, sorrow, and lamentation will not benefit departed relatives in any way. They will remain in the ghost world no matter how much we cry.
Great King, the merits shared from the donations given to the noble disciples of the Buddha will be received by the departed relatives right away. They will enjoy happiness for a long time. Sharing merits with departed relatives is a very good habit to develop. You have respected departed relatives and supported the monks. By doing this you have collected much merit which will result in extraordinary happiness for a long time, great King.