How can I limit the approvers field in a team-managed JSM project?

We have a request form that requires an approver, but the correct approver is dependent on the state the request is coming from. Ideally, the requester would be able to select the correct approver for their state from a dropdown list that contains only the people authorized to approve the request. The built-in approver field that is generated when adding an approval step to the request type workflow allows the user to select from any user associated with the space.

I attempted to follow the instructions in the forum post linked here to add a custom user picker field with a filter, but I am unable to select that field (named Chromebook Request Approver) as the Approver field in the workflow editor.

I suspect this is due to the fact that our service space is team-managed, not company-managed. Is there any way to accomplish this in a team-managed space?

I am open to creating an automation with an if-else if that's what it takes, but I haven't found an easy way to apply conditional logic to the approver field either.