Doing that will replace it with existing Admin menu item. You should try instead "/global/cms/admin/mycustommenu" so that it extends within the admin menu and it should appear.
Take a look at this blog post - https://world.optimizely.com/blogs/mark-stott/dates/2021/8/extending-the-admin-interface-in-optimizely-cms-12/ However, I am unsure if you could add a sublevel item beneath the existing subitem.
I've found the documentation for adding MenuItems:
https://docs.developers.optimizely.com/content-management-system/docs/adding-and-configuring-menu-items#extend-the-user-settings-menu-using-menu-provider
How ever. How do add a menu item for the Admin > Administer Users -> [USER]?
Which path is this? "/global/..."
In the old CMS 11.17.x you did this:
/global/cms/admin/[USERNAME]?
You should try instead "/global/cms/admin/mycustommenu" so that it extends within the admin menu and it should appear.
Take a look at this blog post - https://world.optimizely.com/blogs/mark-stott/dates/2021/8/extending-the-admin-interface-in-optimizely-cms-12/
However, I am unsure if you could add a sublevel item beneath the existing subitem.