Show only custom Action under MSS actions

Summary:

Show only custom Action (Extend Contract Dates) under MSS actions.

Content:

Contract start dates and end dates are assignment DFFs. Each year, Line managers extend or end date the records based on requirements. To accommodate this, we want to grant Line managers access to update assignment DFFs using self-service. We've introduced a custom reason, "Extend Contract Dates," under the Change Assignment type for this purpose.

Despite creating separate Design Studio rules for HR and Line managers to control visibility, Line managers can still see other actions under Change Assignment, such as Demotion, Assignment change etc.

How can we restrict access solely to the custom reason (Extend Contract Dates) so that Line managers can only access contract dates? or is there any other solution to achieve this requirement.