In order for a Hub's Form CTAs to connect with any of your Oracle Eloqua forms, the forms must have the standard Eloqua Email Address contact field (Database Name of C_EmailAddress) added to that form, and that field must be used to uniquely identify a contact.
To do this, you'll need to make sure you have a Processing Step added to the form in Eloqua. Here's how to make sure:
Step 1 - Open Form and Access Processing Stage
Open up your form and click on the Processing button
Step 2 - Add Processing Step
Click on the '+' button to add a new Processing Step and choose Update Contacts - With Form Data.
Step 3 - Select Email Address
Select Email Address (or your Oracle Eloqua Email Field Display Name) as the field that uniquely identifies a contact in the drop down menu, and ensure this processing step executes Always.
Step 4 - Save
Click the Save button and as long as your Form also contains the GUID contact field, you can connect a Hub CTA to this Form.