If you’re hosting on Dashingdon with a game that isn’t compiled, I think all you have to do is include the command:
*sm_init mygame | 3
at the beginning of your first scene file. Then check the little box on the Dashingdon ‘edit game assets’ page to enable the save slot system. I have my first scene file in a different .txt file to startup.txt, so I put the *sm_init command at the top of that first scene.
There are instructions on the Dashingdon ‘edit your game assets’ page, too. And then you’ll be good to go with save slots!
Will that suit, or do you need it to work in a compiled game? Or did you want to have specific save points? If so, you can also have a look at this post.