How to add Dropbox to Thunar's sendto menu

First in your home's hidden directory in the folder named ".local" within it's subfolder named "share" create a folder named "Thunar" then within that folder create a subfolder named "sendto" 

Example:

/home/user/.local/share/Thunar/sendto

Now open your text editor and copy and paste the following:

[Desktop Entry]
Type=Application
Version=1.0
Encoding=UTF-8
Exec=cp -r %F /home/"user"/Dropbox
Icon=dropbox
Name=Dropbox

Next replace (above) "user" in '/home/"user"/Dropbox' with your user name.

Then save the file as "Dropbox" within the "sendto" folder, now close and restart Thunar.