Limit workspace users choice to 1 user instead of multiple (like the Assigned to) field

I need to add the workspace users as a custom field in my template and currently it doesn't limit to just one user, it allows multiple users which I cannot have. Can we have an option to make it a single choice only.

1. in the attached graphic is the out of the box Assigned to (1 choice)

2. in the attached graphic is the custom Architect Assigned to (allows more than 1 choice) - this needs to be limited to just one choice like the out of the box field.