When you are creating custom connector in the definition part where you will define the Actions. You need to define this variable in your request such as
After you have created your aciton and test the custom connector. Simply add the custom connector as a data source in your power apps and this action will be available to you. And you can use the Formula bar to pass the parameter to action.
I have just shown you an example of GetTask connector similarly you can crate your GetUser action in the custom connector and then call it from PowerApps.
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