How would I add value from Table "A" Cell "1" to Table "B" Cell "2" based on a value in Table "A" Cell "3" that matches a value in Table "B" Cell "4"

Question

I really don't know how else to phrase that question

I have two tables, a "Call" table and a "Unit" table. Rows are added based on responses to a form above them.

The "Unit" Table has the "Callsign" and "Assign" columns. The "Call" Table has the "Call Number" and "Units" columns.

If the value in a row's (Row A) "Assign" cell matches the value in a row's (Row B) "Call Number" cell, I would like to add the callsign value from Row A to a list in the units value from Row B.

jsfiddle can be found here: https://jsfiddle.net/TheGerd/d8pfseuc/#&togetherjs=4CbJ54GQzj

I don't entirely know where to start with this. I've used javascript to correlate values in "callsigns" to options in "primary unit", but I don't know how I would add multiple callsigns to the units row without deleting callsigns that are already there.