On the desktop site - this is the link:
You could try copying that and pasting it into the address box in your browser on mobile.
Otherwise, in the browser on your mobile device, when you arrive at the Halo Support site, scroll down and just above the heading ‘RECENT ACTIVITY’, you should see ‘How to Submit a Halo Support Ticket’.
Tap on that and scroll down until you can see ‘RETURN TO TOP’. Just above that, you should see ‘Ready to report an in-game issue?’ - the link is the ‘Submit a ticket’ text - tap that. I don’t know what to do past that as I’ve never submitted a ticket.