Ali,
Found this for you hope it works.
email your Dollar Express number along with your holiday details to DOLLAR@DTAG.Com and they will add your express number to the reservation
if this works then all your details will be with dollar, so you will be pre registered.
This is bit from the TCD website, i know your not booked with them, but i thought the info would be the same.
Voucher / Reference Number - Enter your TCD booking reference (e.g. 123456/S06)
Dollar Confirmation # - Leave this blank
Renter name details - You will need to enter the lead passengers First Initial and Last Name
Pickup Date - Enter your holiday start date
Location of Rental - For most TCD passengers flying to Orlando this will be Sanford Airport, if this is the case simply choose "Sanford - [SFBP01]" from the dropdown list