If you would like to set up a required role of a transaction please navigate to the Transaction roles section of the Fields tab and select the role you would like to make mandatory. The role can be set as required depending on the transaction status, transaction type, or side represented.
Given the condition set for the transaction role, on the third step of the transaction creation process, you will be asked to enter the required contact. Create button will become available only after the required contact is entered.
You can also set default contact on required transaction roles by selecting "including email" and typing the required contact name.
Then this contact will be automatically assigned to the Contacts when creating the transaction.