Hi Nelson,
The Thing Type for Service Request is correct, but we do not support direct navigation using Internal ID yet for service request like we do for other objects, this has been added to backlog.
The alternative way of building a direct navigation link is using the technical ID (UUID) for service request, give this a try and see if this will work for you.
In the workflow rules, one of the available tokens is Technical ID
Example:
Regards
Sushant