How to create a role that shows up in the "Custom" section of the Project Permissions screen when adding a member

How to create a role that shows up in the “Custom” section of the ‘Project Permissions’ screen when adding a member to a project.

We want a set of three different project roles. When we add a member to a project, each combination of those three roles is valid (eg. role1 or role3 or role1+role2 or role2+role3).

What happens now is, that our custom role pops up as if it was a Owner, Member or Read Only role.

How can we accomplish this on rancher v2.6.12?