To trigger a flow from any control of your powerapps (as a trigger) just go into your powerapps interface, create a button to start a flow (not yet here), click on Last Tab "Actions", then on Flows. Let that Button selected. Then click on the button in the right pane "Create a new flow". There a some PowerApps Flows there and above of that list there is the option to let this linked with your button to trigger it.
I ve seen this on this video here, go to Minute 29:30, where the next steps are also explained... Hope that helps. Greets, Eva.