Connect fields of modules

Connect fields of modules

Hi, I have created a custom field inside the customer module, it's a select field. Now I would like to include this field also into the invoices module, but the field should not be editable inside invoices. I just want it to show the value of the field that was selected inside the customer. So basically the two fields must be connected.

Is that possible? I haven't found a way to do this yet.