Hi @John Warner!
There is a out of the box Workflow 2010 with request sign off, ive seen that in classic SP 2013 sites. Last year there was an article about introducing the new request sign-off flow.
Pls find that on this page.
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.