Hello thank you for your thread. To use this Ticket you must have Zekrom in your account, if you haven't make sure to complete it Zekrom Guide . After you have completed, you can utilise the ticket by interacting with Psychic Marina in Vermilion City. Keep in mind that the NPC will identify it as a normal Reroll Ticket , but your Zekrom Reroll Ticket will be used instead. So when using the ticket, use it as if you were using a normal Reroll Ticket. Furthermore, this ticket cannot be used in the Double Reroll Service, which in simple terms, refers to Rerolling your Legendary Pokemon ivs (in this case Zekrom) twice using 1 Reroll Ticket. Hope this has helped you with your query.
Divinelight