I want to create a Customer form which contains a multi-select Customer field. When I try to create such a relation, I can't find the Customer form in the list of available forms to choose from. See attachment.

What I'm struggling with is, some of my customers use different emails address to place orders. Sometimes they place orders themselves, some times their assistant places them. I want to have a way to group the related customers together. That's why I want to create a Bidirectional Customer-Customer relation. Any other suggestion?