Thunar-Dropbox is a plugin for Thunar that adds context-menu items from Dropbox. It was originally developed by Maato.
Depending on your distribution you might have to install libthunarx-3.
The plugin is available in the repositories of most distributions, check it out here:
cmake -B build -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX:PATH=/usr .
cmake --build build
The first call to cmake configures the build system. The -B flag specifies the output directory, here build. The dot at the end is the path to the directory containing CMakeLists.txt.
DCMAKE_INSTALL_PREFIX sets the installation directory and might be unnecessary on your distribution, or you might need to set it to some other path.
The second call actually compiles the project into the build directory. Afterwards, you can execute
cmake --build build --target install --config Release
to install it to the proper directories.