You can create your own plugins using the supplied plugin template (exApiTools.Plugin.Template).
To do this, first install the template on your system:
dotnet new install exApiTools.Plugin.Template.2.0.0.nupkg

Then, in a folder where you want your plugin sources, run:
dotnet new exApiPlugin -n MyPlugin
A folder with all the required files will be created, study its contents and develop your plugin.
To build your plugins in an IDE, you might want to set the environment variable "exapiPackage" to the folder where your instance of PoeHelper is.

When you want to test your plugin with HUD, simply put in Plugins/Source like this:
Plugins
|
->Source
  |
  ->MyPlugin
    |
    ->MyPlugin.csproj
      file1.cs
      file2.cs
      ...
After this, launch the HUD and you'll either be presented with compilation issues or the plugin will load.