I've just realized the new 'Manage Steam List' page shows some text that looks like a leftover from the 'Add Sale' page:

If a game/bundle is not in this list then our Steam sales importer will not import it, the Steam API checks against this list if a game is on sale to include it or not.

Here you can add a Game Sale Provider. As in where the sale is located.
Game/Bundle Name, Must be the EXACT name as on Steam

Plus, when hovering the mouse over the text fields' headers, the pointer changes as if it were over a button or a link (it's odd though, 'cause if I try to make a screenshot the mouse pointer appears normal). Investigating further, I see the same happens to some of the text fields in the 'Add Sale' page, namely the price ones.