Before beginning, generate the code required to integrate your button: see here for info (part 1).
Login to your to your Wix account.
Click Add Elements > Embed code > Embed HMTL
Add the full code (all parts) in the appropriate section
Place your button where you want it to be
Preview your page to test it out
Wix does not allow the widget to display normally (i.e. the widget window is limited by Wix's element dimensions). For this reason, we recommend selecting the option "Use as popup" (add "usePopup: true" under the API key in the body tag) to avoid this issue.