You can make sure HelloModdingWorld can find all of `.dll` files it needs by copying your Techblox folder here and renaming it to `ref`, but you'll have to re-copy it after every Techblox update.
You can also create a symbolic link (look it up) to your Techblox install folder named `ref` in this folder to avoid having to re-copy files.
For any mod to work, you will have to patch your game with [GCIPA](https://git.exmods.org/modtainers/GCIPA).
[Direct link to install guide](https://git.exmods.org/modtainers/GCIPA/src/branch/master/README.md#how-to-install).
This project also requires the [GamecraftModdingAPI](https://git.exmods.org/modtainers/GamecraftModdingAPI) library to be installed (in `ref/Plugins/GamecraftModdingAPI.dll`).
[Direct link to install guide](https://www.exmods.org/guides/install.html).
If you don't want to use the standard modding API,
you can remove the dependency by removing `<Reference Include="GamecraftModdingAPI"> ... </Reference>` in `HelloModdingWorld.csproj` and removing all mentions in `MyPlugin.cs`.