If you refering to the page where you can buy a Steam key on Itch, you have to claim the Steam key in the "My Library" at your profile from the top right.
-Click on the downward arrow of your profile and hit the My Library.
-Then navigate to your Sapphire Safari Steam key and hit download.
-From the download page there is a butten where you can claim the key.
-Once you click on that, you should be getting a code to use as a key, so copy it or write it down.
-Then you can redeem it on Steam by going to Steam and clicking Add a Game, and select Activate a Product on Steam.
If you however bought the Sapphire Safari Early Access instead and hoped you would get a Steam Key included, then you are sorely mistaken, because they dont sell them as a bundle. There exist two types of purchase on this site, and thats Sapphire Safari Steam Key, and the other is Sapphire Safari Early access.
Viewing post in Sapphire Safari (Beta Access) comments
What i said was that there is two types you can buy on this site, and one of them gives you only a Steam Key, while the other the game for Itch only.
There is no way to buy the game for Itch and get a Steam key included with the purchase.
Ofc right now they not available for purchase because of issues with their itch store, so its tempoary down.
So if they want to buy it for Steam, they have to do it directly at Steam store.