In your AddRow formula, where you set the lookup column for the new row in table_2, put the AddRow math here instead.
In my example the column looking at table_2 is called T2.
What it will do is create the row in table_2 and automatically reference it as the lookup for the column value.
This can be useful in other situations. For example, create a lookup field somewhere on a blank page that references a table, then use SetControlValue in conjunction with AddRow to both set the lookup as it creates the row. This control can then be referenced multiple times throughout the code whenever you need to look at the most recently created row.