Linking an Asset to a Service Request Ticket from within a work flow

Hi,

I want to link assets to a Service Request ticket using a workflow.

Scenario: in a Service Catalog item i added a custom filed, that allows the requester to choose from a certain asset type  (e.g. printers). 
so i do have the name of the printer available in a custom field for the Service request. what action or method (maybe API call?) can i use to make an association of that Asset to the ticket?

any ideas?

thanks,

 Günther