You did an .exe that marshals the inventor Application ?
Basically, you use the AddinServer class (just like the example in the SDK). in the activate method of this class, Inventor passes its AddInSiteObject which contains the Inventor.Application. From that point, your code should be almost identical.
You then place your compiled .dll (class library) along with the .addin file in either
C:\ProgramData\Autodesk\Inventor 2020\Addins
or
C:\%USERPROFILE%\AppData\Roaming\Autodesk\Inventor 2020\Addins
ProgramData bring the addin to every user on this computer, but requires Admin access, AppData is user specific.
When opening inventor, it will detect the new addin and ask you if you want to allow it (you can alway browse addins from the Environments ribbon later)
Note: Use the .addin file from the SDK example to guide you. It will help you understand to match ID, GUID and assembly name.
Also, if you wish to run in debug mode, go to your project properties: in the debug section, select Start external program and point to something like : C:\Program Files\Autodesk\Inventor 2020\Bin\Inventor.exe
in the build section, select the output path to be where the addin needs to be (if ProgramData you will need to run VS as admin). You need Inventor to be closed before starting to debug since it will overwrite the dll that would be otherwise write-protected by Inventor.
