Answer:
Foot-in-the-door technique.
Explanation:
This technique involves having an individual do something small so that, in the long game, you manage to convince them of doing something bigger. In this case, wearing a button that says "adopt a dog today" would be a small feat to accomplish. After some weeks of wearing that, you ask this same person to adopt a dog. In this scenario, you would have greater chances of convincing them if they wore the button.