I know it can be done with both Postman (because I've done it long, long ago) and with Power Automate (because I've known people who have done it).

My first question is one of semantics. What flavor of Alteryx Gallery are you working with? (Alteryx has a bad habit of using words repeatedly to mean a variety of different things.) Does your company have Alteryx Server?

Have you been able to initiate a workflow from the Swagger documentation page? (https://baseURL/webapi)

If you have, then it is just a matter of figuring out how to do appropriate authentication. That is more challenging than it seems. (I used to have a workflow from one of the best guys on the Support team who did it for fun, but that was before OAuth2.) For the Power Automate, you can use the Power Automate tool. This might be a good starting page for Postman adaptation of your calls.