The New button would create a new record that would link to the parent record you started from. If you absolutely need a button that is named Add, you could copy the functionality of the New button into a custom UI action. You would build this UI action on the table that is being shown on the Related list. The Related List shows UI actions from that table. To find out the table, you can right click on the query in the Relate List, in this case it's Parent Incident = INC00473805, then click open in a new window. This will open another browser tab with that table.
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