On njrcna.com we have a "Click here to Book a Room" announcement with Cistom URL
https://www.marriott.com/event-reservations/reservation-link.mi?id=1745862806501&key=GRP&guestreslink2=true&app=resvlink
When you click the link it instead returns
https://www.marriott.com/event-reservations/reservation-link.mi?id=1745862806501&key=GRP&guestreslink2=true&app=resvlink
It adds #38; after each & character which causes an error on the destination website. It only does this when clicking on the link in the banner (we did not try modal) - it does not add these characters when visiting the custom link from the Mayo admin page using the "Open link" button.