Add an html checkbox in OTRS ticket submit page

I'm new to OTRS and I have to add an HTML checkbox with some text in the ticket submit page. Essentially, I need the user to accept a privacy statement (clicking on the checkbox) before he submits a new ticket. Hence, the submit button has to be enabled only after the user clicks on the checkbox. How can I do this?

I know HTML, but I don't know OTRS architecture so I can't figure out how can I modify HTML pages.

In Kernel->Output->HTML->Standard I've founded HTML templates (*.dtl), but:

1. I don't know which elements are part of ticket submitting page;
2. I don't know if I need to modify an existing template, adding HTML, or to create a new template (in this second case I think I should modify some perl module that call the layout, but which one!?!)
3. I'm trying to open .dtl files to understand which I've to modify, but Firefox can't render them correctly; how can I open them?

If you want customers to accept a privacy statement you don't need to code.

You can simply go to Admin > SysConfig > Framework > Frontend::Customer and activate the CustomerPanelPreApplicationModule###CustomerAccept module, and the InfoKey and InfoFile values. When the customer logs in the text in the InfoFile is displayed and the customer needs to accept this. After accepting the InfoKey is stored in the User Preferences. This way the system will not bother asking permission to the customer again, and if you might change the policy you can change the key as well, displaying the message when needed.

For your questions on .dtl files: these are plain HTML files, but different blocks are rendered by the files located under Kernel/Modules.

If you want to modify the page that is used for submitting new tickets, it's this file: CustomerTicketMessage.dtl. You can edit these files with any text editor.