If you trigger this from a button, you can set the property ShowProgressBar of the Action Button, see the documentation here.
As suggested by Rom, if the API is called from click of an action button than you can set the show progress bar in microflow setting to ‘blocking’.
If you have some custom UI and want to show something e.g. a spinning wheel than you can use microflow timer widget and add custom logic to achieve this. You can use your own .gif or ‘Running’ gif from System > Images, add conditional visibility on the page and refresh visibility in the logic in microflow timer widget.
Hope this helps!