The ”FM hookup devices” were usually just a cable splitter with one leg connected to your FM radio. Systems that had the MTV stereo on FM usually already had a lineup of distant FM stations and satellite delivered audio signals modulated on the FM band.
When I lived in Amarillo, Texas in the late 70s-early 80s the cable system imported seven stations from DFW which were receivable on FM, as well as WFMT out of Chicago. When MTV was added as a TV offering, its stereo soundtrack was available on 96.1 MHz.
The cable system later added a premium audio service that still used FM, but with a converter vox. For this a number of satellite delivered audio services were modulated between 114 and 120 MHz. The converter box allowed them to be received betweem 90 and 96 MHz. The MTV stereo audio was moved to this premium service, which also had stereo soundtracks for HBO and the Movie Channel, as well as KKGO in LA.
The advent of stereo television in the mid 1980s made the MTV stereo hookups obsolete.
I do recall MTV promoting the stickers, but I think most people could easily remember the frequency.