A game whose catalog entry lists Modrinth or itch.io gets a chip for it beside All sources, and its results join the merged search. Stardew Valley lists neither.

Modrinth needs no account. Add installs the newest version with its required projects.
itch.io needs your own API key: create one in your itch.io account (Settings › API keys), paste it into API key in Settings › Accounts and choose Test. The key is kept in your system keyring; Remove key forgets it. Until then the chip is greyed out. itch.io has no sort order or categories, and Add takes the game's first upload that is not a demo or soundtrack.