Customize the "Support Ticket" Form

Customize the "Support Ticket" Form

Hi there,

we are using ZOHO One and i would like to know, how I can customize the fields of the "Support Ticket" Form in the Helpcenter? (https://desk.zoho.eu/portal/xxxxxxxx/en/newticket)
I want to add some custom fields to the form which are mandatory. 

I found a way to create different ticket templates, but i didnt found a way to customize the fields of these templates.... 
Can anyone help?