To do so, use the "Email us" button situated in the Parameters menu.
It is essential that you do not change the text that is already filled out in the email for you.
This number is your Account ID and it is the only way customer care can help you with your issue.
Customer care will require the account IDs of both your clients to do the transfer.
To get both of them, you need to log out (or login) and then write down the number automatically entered in the email form for you.
After you send your request, the customer care should get back to you shortly after.