Hi Heather,
Hope you are well!
If you have imported the values of the custom field for the registrants, you can simply run the 'Registrant Details' report within the event and add the custom field in the report to get the data.
For the people coking in via open link, you can export the list of the registrants later once they have registered and create an excel file with their name, email address and the custom contact field and its values for everyone and import the file in the address book that will update the custom contact field for all the registrants who came in from open link.
The custom contact fields are saved within the invitee's contact record within the address book so these fields will always be available there if you want to manually update the value for any custom field for a few invitees as well.
You can refer to the following articles for your reference :
Using Custom Contact Fields == LINK : : https://support.cvent.com/s/communityarticle/Using-Custom-Contact-Fields
Importing Event Contacts == LINK : : https://support.cvent.com/s/communityarticle/Importing-Event-Contacts
I hope this helps. Please let us know if you have any further questions.
------------------------------
Raashi Semwal Khati
Senior Quality Analyst
Cvent
------------------------------