Hi Peter,
If you cannot use the blocking in your microflow settings, perhaps you can use the javascript actions showProgress and HideProgress from the nanoflowCommons module?
Hope this helps!
If you call this integration from a microflow button on a page you can set a blocking progress bar in the microflow settings, it will just show a progress bar until the microflow has finished.
The problem with answers from Johan and Glenn is that I want the progress bar to be on the page, not as a overlay on top off the page.
I have added an idea to the community to implement this in the pogress bar widget.
Please give it a vote if you like the idea: https://forum.mendix.com/link/ideas/3474
One way to do this can be to have a progress bar in the page. You can program the progress to behave like an indeterminate bar by setting the values to 0 initially and increment that to 100 while the rest call is in progress and repeat this as long as the api call response is not received.
https://marketplace.mendix.com/link/component/48910