A Sticky Benjamin falls into the same category as a "Dirty Sanchez," a "Rear Admiral" or a "Cleveland Steamer." Bascially, you ejaculate all over some portion of your pay-for-play partner and then take a Hundred-Dollar Bill and slap it over the ejaculate, making it stick to whatever part of the body you've spooged on. Depending upon the denomination you're using, this could also be a "Sticky Washington," a "Sticky Lincoln," a "Sticky Grant," etc.
Shaking hands with, or other otherwise touching, an unsuspecting person directly after you have masturbated and gotten it on your hands. Cum may be rustified, or still wet.