Please follow the below steps to display Primary Contact First Name in Email messages.
Navigate to Customization > List, Records & Fields > Entity Fields > New.
1. Complete the Label for your custom field. eg. Primary Contact First Name.
2. Set Type = Free-Form Text.
3. Under Applies To subtab, check Customer.
4. Under Display subtab, set Subtab = Main.
5. Under Validation & Defaulting subtab, check the Formula box.
6. Set Default Value = {contact.firstname}
7. Click Save.
Now add your field in your email template.
The syntax is ${customer.yourcustomfieldid}.
Perform a test email to ensure only the Primary Contact First Name is referenced.
