Hi Larry
AFAIK, there is no standard feature in the REST adapter to generate such a signature.
I think you'd need to generate the signature using some Java logic. Some API providers provide sample logic to access their services, like the example for Amazon below. You can probably check with the provider if they have something similar.
Java Sample Code for Calculating HMAC-SHA1 Signatures - Amazon Simple Queue Service
You can use third party libraries to do so, for example the open source library from Apache below. I haven't tried it myself, but you should be able to search for examples online.
Rgds
Eng Swee