Add custom text to "Submit a request" form on Zendesk Guide

Hi all

We need to add some custom text as information in our Submit a request form page on Zendesk Guide. 

Pretty much we want to add information telling customers to check their spam inbox if they did not receive any reply from us, since there seems to be some cases where this happen.

How can we modify the form page and add this information?

Screenshot of our form attached. We would like to add the information in relation to the Submit button.

Thanks