You would appear to have these tables and relationship:
Clients::id = Appointments::ClientID
Define a field in Clients, named SelectedClient of the same data type as Clients::id.
Make a new table occurrence of Appointments named Appointments|Selected.
Add this relationship:
Clients::SelectedClient = Appointments|Selected::ClientID
Put a Button in the portal row of your portal of appointment dates and set it to perform this script:
Set Field [Clients::SelectedClient ; Appointments::ClientID ]
On your tab, remove the current portal and fields from it.
Replace them with fields from Appointments|Selected and when you click a button in your portal row, the fields on your tab will display data from the selected Appointment Record.
It worked like a charm (i only had to make a slight change, by relating the selected_client field with the Appointments_ID_FK)
thank you very much, great and clear answer!
Good catch on my mistake! It also confirms that you understand how the relationships and table occurrences used work.