You can’t make source changes and also sync it with the marketplace. When you use a launcher build of the engine, plugins in the engine folder are prebuilt.
The easiest approach is to copy the plugin folder (from Engine/Plugins/Marketplace) and put in in YourProject/Plugins. This will require your project to be a C++ project though, so if it’s not already you’ll need to first add some dummy C++ class to the project. Once you have the plugin within the project folder hierarchy, it should be rebuilt with the project. You may need to uninstall the marketplace plugin after copying it, I’m not sure if the build system can deal with the same plugin in two places.