I've set up a custom print service that has a number of templates added from ArcPro. In ArcPro I set the scale for each of them and added a scalebar. When I go to use these custom print layouts in the print widget for WAB it uses the current map extent rather than the one set in the template. I know that the widget gives you the option to set the the map scale, map extent, or to manually enter a scale, but is there any way for the widget to automatically set the scale to be the scale from the print template it's using?
Unfortunately the answer is no if you go with out of the box solutions. Layout Templates and Print services are meant for generic use.
If you need to restrict to any specific scale, then I'd recommend you to create an arcpy based print service and have that map scale restrictions in the arcpy code.
As you know in Pro, the Export Web Map gp tool is a script tool. You can use that arcpy script associated to this tool as a starting point.
here is a tutorial on 'how to publish an arcpy based print service' - only thing is that it was written using ArcMap.
But we are planning to add another tutorial to describe how to do the same from ArcGIS Pro - I'm hoping it will come out very soon.