How to link a contact to a BtoB contact ?

Go to the Contacts tab (1) and search for your contact (see Search for a contact). Once you have found the contact, check the box to the left of their last name to select it (2).

Once the contact is selected, go to More options (3), then click Link to an organization (4).

A window will then open. Click in the field to display all the structures you previously created, then select the desired BtoB contact. Finally, click Save to link the contact to the structure.