Hi!
I’m not very familiar with Harlowe, but looking at the documentation (Harlowe 3.3.8 manual), you do not have to type out every variable you want saved - the save macro records your complete game state, all story variables included. So the correct code to put in the “header”-tagged passage would be something like (save-game:"Slot A", "Test")
You can then put a link to load the game from the save slot wherever you want, like on the opening screen (the load-game is also described in the docs I linked): (link: "Load")[(Load-game:"Slot A")]
However, since putting the code in the header passage means the game is saved in every passage, this will cause an issue where loading from the opening screen just reloads said opening screen. To avoid that, you can create a tag for passages where you don’t want to save and wrap the code in the header in it:
(unless: (passage:)'s tags contains "menu")[(save-game:"Slot A", "Test")] (the game won’t save in “menu”-tagged passages).