Source Primary Contact into the Custom Entity field applied on Customer record type

1. Navigate to Customization > Lists, Records, & Fields > Entity Fields > New.
2. Enter the name of the field on Label = Ex. Primary Contact.
3. Set the Type = Free-Form Text.
4. Set Store Value = F (not checked).
5. Under Applies To Tab: tick the check box on Customer.
Note: This can be also applied to Project.
6. Under Display Tab, select Main from Subtab dropdown list.
7. Under Validation & Defaulting Tab, tick Formula check box then set this formula as Default Value:   {contactprimary.entityid}
8. Hit Save button or Save & Apply to Forms.

 
Note: This is a possible workaround for Enhancement 125764.