By default, your Treefort app includes a contact form which allows users to enter a subject and message and send you a note from within the app to the email address you've specified. The benefit of this is that the contact form can collect certain diagnostic data such as app/browser version and include that metadata in the message. However, sometimes you may want to provide a more personalized form to enable users to contact you - for example, a web form that could contact you using other methods besides email.
If you wish to self-host your own custom contact form, then you'll need to set up publicly-available site on the web. (Make sure to test your form to make sure it works well on on desktop as well as mobile!) After you have the URL of your new form:
Go to App->Settings in your dashboard
Enter the URL of your contact form in the "Custom Contact Form" field
Click "Publish All Changes" to publish your manifest.
After that, once the app retrieves your newly-published app manifest, clicking on the "Contact" link in the account menu will embed your custom form in the app in place of the default form.
For extra customization, custom contact forms also support all of the placeholders supported by web embeds.