Using automation to duplicate between boards - missing information

Morning All,
I’m using an automation to copy infomation from my “main board” to some smaller boards. They are sharing information both ways and then on the main board it has mirror fields to pull in the data from the smaller board.
I’m using “Job Number” as a master ID however when its copied to the smaller board i have to manually select the job number in the linked field so that the mirror column in the master is shown.

How can I specify that they are linked so when the automation creates the new item in the small board it already uses the data to create that link?