Using Communities Template - Trying to add a multiselect to the signup form

I am using the Communities Template. On the Signup page, I am trying to add a multiselect field which can pull from the multiselect field ‘Spaces’ which is a field in the Signup table. The ‘Spaces’ field pulls from the ‘Spaces’ table for all the available spaces to choose from. Is it possible to do this? I see in the layout there is an option that is greyed out ‘Sync options with data source’. It says only multi-select, single select or linked to record in AirTable will work for this. In the Airtable table for ‘Signup’ table, it has this ‘Spaces’ field as a link to the ‘Spaces’ table records. So I do not see why this does not work?