This drove me crazy, too. But I figured it out... at least for me. I had to log into my PreSonus account and then go to the entry for my version of Studio One Artist (I know, even though I have Pro, that didn't show up, but Artist 4.6.2 did). There on the Artist page was displayed the fact that I was entitled to 4-free months of Sphere. From there, it was smooth sailing. Needed to enter a credit card but that was only to charge after the four month trial was over (if membership was to continue).
Poke around your user account and hopefully the green button will be there in your Artist account.