Answer:foot in the door
Explanation:
Foot in the door is a persuasion technique in which a small request is made knowing that if a person agrees to a small request it is likely that a person won't refuse when a bigger request is made.
You can think of this as when you have your foot in a door it is likely that the owner of the house won't shut the door once the foot is in.
Having people sign a small petition is a small request that no one can refuse which opens a way to the bigger request to ask for donations. After people have signed the petition it is likely that they won't refuse giving out the donation.