Custom Table that References CustTable

I am new to development within AX 2012. I am working on creating a custom table that will store user-defined fields. This information is tied to customers, so it relates to the CustTable table. I have the relation set-up and everything works fine if I enter the information in AOT. However, when adding the information on the customer form it is not grabbing the CustAccount. It only writes the value of the user-defined field. I was wondering where I need to write the code to insert the CustAccount number. Is this done on the form?

Thanks.