Hey! I know this is an old post but I’ve been working on this exact same issue and wanted to share what I’ve found so far since there’s almost no documentation on it.
The contextual menu buttons are driven by components on the Pawn you are using. Each button you see in the menu corresponds to a component at the bottom of the component list in your Pawn (Snapshot, Dimension, Annotation, Bookmark, etc.).
To remove a button: Simply remove the corresponding component from your Pawn. No component, no button in the menu.
To add a custom button:
Find the component Blueprint of an existing option (e.g. BP_BookmarkComponent) in the Collab Viewer content folder
Duplicate it and rename it
Modify the duplicated Blueprint — in Event Execute After Spawn Pawn you set the button name and icon, and from there you call Create Activate Button which is the function that actually creates the button in the menu
I’m still working on adding sub-options to my custom button, but in principle it seems to follow the same flow as the existing components, so that should be the right direction to look into.