Within Teamwork Desk you can choose to include the ticket number assigned by Teamwork Desk in the subject line of your reply to a customer.

The ticket ID number will appear within brackets in the subject line as below:

'[83976516] I hope you can help with my issue...'

To enable this, click on the your profile icon to the top right of the navigation bar and select 'Settings:

Near the top of the page select the 'Company' area of settings:

Under the 'Message Settings' section you will see an option to enable 'Include ticket number':

Once this is enabled, the ticket number assigned by Desk will be included in your reply to the customer which can be used for reference by them.