There is no built-in context-menu in the scheduler, it is expected that you connect some 3-rd party tool.
There is an API event
onContextMenu which fires on the right click and which you can use to show a menu.
In our samples you can find an example with dhtmlx
Here is a related sample:
Note that this context menu is not a part of
dhtmlxScheduler library and is not available separately from DHTMLX Suite.
You can use the free version of DHTMLX Suite under GPL license, otherwise, you’ll need to obtain a commercial license of the component