To hide the button when the row is not the active portal row, select the button. In the Data 
tab of the Inspector, enter the following formula for Hide object when:
Returns a number representing the record that has focus in the current found set.
Get(ActiveRecordNumber)None
number
FileMaker Pro Advanced 17.0
The value returned is the current record number displayed in the navigation controls in the status toolbar. It is determined by the relative place of the record in the found set, and it changes depending on the find criteria and the sort order.
•To return a value that uniquely and permanently identifies a record in this table, use the Get(RecordID) function.
Returns 4 when the fourth record in the found set has focus.
In a portal that shows records from the current table, suppose you want to display a button in a portal row only when the row is the active portal row. (This condition occurs when the record’s position in the current found set is the same as the number of the record that has focus.)
To hide the button when the row is not the active portal row, select the button. In the Data tab of the Inspector, enter the following formula for Hide object when:
not ( Get ( RecordNumber ) = Get ( ActiveRecordNumber ) )You can also apply this example to show a button only in the active record in List View.
See Creating portals for master-detail layouts and Hiding or showing layout objects.