See explanation below. Explanation You need to create a customer lookup field then add it to the Opportunity entity. Go to Settings > Customizations. Select Customize the System. Under Components, expand Entities, and then select the Opportunity entity. Select Fields. Click New. Create the Billing Contact field. Set the Field Requirement to Business Required. Set the Data Type to Lookup. For the Target Record Type select Contact. Click Save and Close. Click Publish All Customizations. Open the Opportunities form. In the Field Explorer drag the new field into the form. Click Save Click Publish