How to save Entries to an entry? (non-plugin)
All related fields need to be passed as an array even if it is limited to 1.

so for example change your eventBooking and userBooking hidden fields to be the following (also you may need to specify the user id when relating to a user):

&lt;input type="hidden" name="fields[userBooking][]" value="{{user.id}}"&gt;
&lt;input type="hidden" name="fields[eventBooking][]" value="{{eventID}}"&gt;


This should get you want you need.