Presently, there is no easy way to do this on the web. Editing related records and features should be on the roadmap for future editing tools built in the JavaScript API 4.x , so you should see this eventually in the new Map Viewer and in Experience Builder.
Perhaps, another way you could get around this limitation, would be to have a "project area" polygon layer. The polygon layer would have a field for the key ID you are using to relate your features.
You could then use an Intersection Attribute Action in the Smart Editor to get the ID from the polygon for all the features that you place within the area. In this way, the ID will be automatically populated into your related features, which will prevent user error and be less tedious.
If your "project areas" overlap, you could use the Value Picker to pick the key ID (in other words, the relationship) you would want the feature to participate in.
If this doesn't make sense, I can put together a quick example.