It depends on the details of your flow and if any parameters are involved.
For a simple flow try:
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You can try below steps:
1. Open your canvas app in edit mode.
2. Select any screen > Under Action > Click on Power Automate
3. Add the flow in your app for a screen.
4. Pass any run parameters.
5. Now mention the same flow in App > OnStart as well.
6. This will allow you to run the flow when App starts.
Hope this helps !
Please refer to my above screenshot. It does not show action tab in App(OnStart), but you can still call the flow (if its added in any corresponding screen) in function bar.
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