Here's SteamManifestInjector (SMI)
Here are all its features:
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Inject .manifest files, which can be used only for downloading
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Inject .acf files to have your game detected in your Steam library
I'm not responsible for what you do with it, and in any case, I have nothing to do with it.
I obviously don't condone piracy.
Don't try to reverse engineer it. If you want the program code, just ask me, and I'll give it to you without hesitation.
Feel free to support the project by sharing it or talking about it on social media ;)