Download PuTTY executable from http://www.putty.org and put it right next to the MuTTY executable - it will auto-detect that.
The configuration will be stored in your Users Documents directory. In the MuTTY window, navigate to Help > About to find the absolute configuration path.
If you have any SSH Sessions configured in legacy PuTTY, MuTTY will detect them as well, but you will not be able to modify them from MuTTY.
Simply open the solution in Visual Studio 2019+.
Note that when running in Debug configuration, a development configuration file is used (placed in the project directory).
MuTTY uses the DockPanel Suite: http://dockpanelsuite.com/