Since some time Azure allows you to expose your API apps to PowerApps and Flow in an easy way. Just go to your API definition (make sure CORS is enabled) and click on the export-button.
This gives you a dialog where you can download or get a link to your Swagger metadata-file. Download it, and click on Go To PowerApps or Flow.
Importing into PowerApps is the simplest to do.In the PowerApps page, select Connectors, custom and then New Custom API. Just link to your swagger file and click next.
There you go. Flow needs a little bit more work.Go to the Custom APIs for Flow and add a custom API. Again, link to your swagger file.
The extra work is that you can (and probably should for improving usability) provide more information for each of your API functions.
After you've done, start using it in flow and powerapps.