You’ve got two different things going on here:
The checkbox for ‘Enable populating of field values’ on the Pods Feed configuration is typically intended for ‘editing’ the current post, since it would be designed to be actually loading the values from the current post.
In the case of a registration to a conference, you need one form entry (and probably one ‘reservation post’ for every person registering to your conference, so you’re actually only pre-filling some specific fields with values from the conference (the conference ID and the amount of the registration), but the other fields might not have anything to do with the conference and instead have to be populated with values specific to the registration process.
What type of field is workshop_location
? BTW, it’s much easier to discuss this kind of stuff on our Slack Chat at https://pods.io/chat/