There’s currently not a way to do that. The “Describe your issue” section is built-in for all tickets, anything you add will go below it.
Thank you for the quick reply, @jclark. I was afraid that was the case, but hoping it was not. I’d be nice to be able to post some sort of informational message and/or instructions above initial text box, before people start filling in information. Maybe this will change in the future with an update. 
@RCoffey Thank you for submitting your question to our community! 
There is not a way to add that custom field at the very top unless it is a confirmation message. This custom field would not allow them to fill anything out, but rather give instruction.
This would be an enhancement request you can submit here: https://community.incidentiq.com/ideas
Thanks @jclark for the answer!