The Custom Role
The Custom Role in Workspaces is any role that has a set of permissions set by your organisation, rather than one of the existing preset user roles you can find details of in this Knowledge Base.
There is no way for users to produce a custom role for Workspaces. This must be done by the support team for your organisation. If you are a Tenant Administrator (TA), you can raise a service desk ticket asking for the custom role to be created.
Once created, the TA will see this role appear as a selectable option when they are performing user administration activities.