Summary:
Even though you can always set ticket fields after the ticket is created, the reporter would like the option to do so while composing their message. While he was asking about 'open ticket' specifically it might make sense to include this feature in 'send mail' as well.
Steps:
1) Create a couple of ticket fields (global or group) in 'helpdesk setup'.
2) Go back to 'mail' and click 'open ticket' or 'send mail'.
Result:
There is no way to set custom ticket fields, like you can in the 'Properties' tab of existing tickets.
Improvement
Open
[Custom FIelds] Allow you to set ticket fields when opening a new ticket for a customer
