When Radiant Jirachi is Knocked Out by damage from an opponent's attack, using its "Entrusted Wishes" Ability is mandatory. You must search your deck, and you must take at least 1 card if you are able to.
Source: Silver Tempest FAQ; TPCi Rules Team (2022-11-10)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH12_120
SWSH12_120