I have items in portal that cover large areas outisde of our project area. When these items are added to the map, the 'zoom to layer' button zooms to middle of of these layers which is in the middle of nowhere to our users. Question is how can I change the properties of these layers so that the zoom to button brings you to the project area or a custom extent? Is this possible? I've tried manually changing the extent box in the mxd dataframe it was published from as well as in the layer's item properties, but neither fixed the problem.
You could create Bookmark to resolve the issue. This is one option.
Bookmark widget—Portal for ArcGIS | ArcGIS for Server