Hi Ingibjorg, if you have DealEndDate as a custom field, then it should appear as an option in that dropdown. If you've just created the field, I'd refresh and give it a chance to appear.
Also, you're going to need to make sure that DealEndDate is the correct format field (I'm assuming date) for this to work properly.
Dory is right in that custom fields should appear automatically, but you can only use it in your example above if the custom field type for your new field is date. It will not appear otherwise.