How to insert new id or return and insert a duplicate id

I have an insert form with two insert server record behaviors in it that uses two tables. Table 1 has a drop down menu showing existing selections and a text field to enter a new value. This field and form work when entering a new value and correctly shows an error when entering a duplicate value. But, how do I get the duplicate id to use to insert into table 2? How do I enter either an existing value or a new value?

I did not find anything in the WA forum or materials about how to do this, but I did find code on the internet using "if...else" around the insert statement; however, I was unsuccessful trying to adapt it to the WA code. I'm sorry, I don't know enough about what I'm doing to figure this out or whether there is a better way to accomplish what I'm trying to do so I'm hoping you can help me out and point me in the right direction. Thank you.