How to Create a Custom Role
Not currently available. The present Arthur MCP interface has no Roles & Permissions route or panel, so a custom role cannot yet be created or assigned.
Check availability
Open the instance Settings screen.

Check the available sections. The navigation contains Server, Security, Headers, E-mail, and Observability, but no Roles & Permissions entry.
Current limitation
There is no executable custom-role procedure until the corresponding route and permission-management interface are released.
Related
Tutorial video