The steam_api.dll or it's 64-bit equivalent is used by the Steam game to communicate with the Steam Client for a variety of information and functionality. Majority of Steam games require this file.
Which is why all cracked Steam games replace this file with one that allows the the game to launch and function correctly without the Steam Client. Otherwise referred to as Steam Emulator. Majority of Steam games are cracked this way barring any other scenarios that have Denuvo, custom anti-tamper, or online-only Steam games.
Scene groups have their own Steam Emulator, but there have been some community P2P developed Steam Emulators. The easiest and preferred recommendation is to use Goldberg Steam Emulator. You can find it linked in the Mega Thread.
The simplest method, and only method you can use because you deleted the original steam_api64.dll, would be to just replace the steam_api64.dll with Goldbergs's version of the same file. Most games can be "cracked" this way. Then create a steam_settings folder in the same directory as the steam_api64.dll that you replaced. In the steam_settings folder then create a dlc.txt file. Open it with a text editor and paste the following:
1171470=Hat Pack (Trailmakers: Hat Pack)
1183540=Skin Pack (Trailmakers: Skin Pack)
1333570=Trailmakers: Skin Pack 2
1607390=Trailmakers - High Seas Expansion
2054110=Trailmakers - Decal Action Pack (Trailmakers: Decals Action Pack)
2054120=Trailmakers - Decal Expression Pack (Trailmakers: Decals Expression Pack)
2285970=Trailmakers - Airborne Expansion
2579530=Trailmakers - Space Voyager Pack
Copying and paste the above will create a blank line between each entry. Delete it sot hat each line is stacked on top of each other.
And I believe that is it. The other comment mentioned Steamless, but that's only if the game executable has SteamStub which I don't believe is the case here.