I'm in more than one Asana workspace — which one does Relay push to?

Relay checks when you connect. One workspace: it's picked automatically and you'll never see a choice. More than one: a Workspace dropdown appears at the top of the card's settings — pick where pushes should go. You can change it later; switching workspace also resets the project choice (projects belong to a workspace), so re-pick a project if you use project mode.