No supported wat to do this, as confirmed by Microsoft product team.
We could manipulate the YAML to include OAuth while connecting to on-premise was offered as a workaround, but its an undocumented approach.
Did you ever resolve this?
I am not sure about getting the user's token from canvas app. But you can try to simulate the steps of Oauth flow using HTTP connector without creating custom connector.
1. Generate an access token depending on the API setup with HTTP call
2. Using the new token in the header for the actual API calls