At what point in the game do you want this to happen?
The easiest way to do it is to put the MysticTicket in the game somewhere (I personally added it as a held item to an in-game trade pokemon). After you get the ticket, you can then add a character in the game that activates its use. I have a simple script that I added to a sailor right outside of the dock to the SS Anne:
'---------------
#org 0x72DD34
msgbox 0x817F723 MSG_FACE '"I heard there are some\nspecial ti..."
setflag 0x84A
setflag 0x84B
end
'---------
' Strings
'---------
#org 0x17F723
= I heard there are some\nspecial tickets you can use\pto see exotic places.
Here, the flag for the MysticTicket is 84A. 84B is the Aurora Ticket. I just put them in the same script, but you could cut that one out if you don't want it.
Now, I'm not sure but I think you might have to defeat the Elite 4 or complete the Celio stone event before this activates. In which case, if you want it earlier than that, you'd have to set the flag for that as well, but it will screw up later events. So it would get a little more complicated from there.