Well, you're not customizing a mouse button. That is another story and something done with mouse drivers. You are customizing context menus...
Graphisoft does not support this. But, you can enable context menu editing for your Work Environment on the Mac by checking the preference shown in the attached screenshot. On Windows, this is done via the registry editor.
Restart ArchiCAD (if it was running when you edited your preferences) and you'll see Context Menus under your Command Layout Schemes. I'm not really sure why GS does not document or support this, but I seem to remember that the feature may not be fully tested...
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB