Hi @chrisk
Please check this Account Provisioning using ISC Forms & Workflow - Content / Community Blog - SailPoint Developer Community and use interactive trigger instead of external trigger to process the provisioning in SailPoint. I know it may not be the exact scenario for you, but the link gives you a good idea about how it can be done.
Otherwise, if you are able to pull the ticket number from the external source via SailPoint source connector (Direct or generic or flat file) before the role request then you can map that account attribute to an identity attribute value and refer it in AD description during Account Creation operation.