To use the clipboard copy and paste functionality in Guacamole you need to press ctrl+shift+alt which will slide out the clipboard snipping applet where you can then paste the items you wish to put on the remote desktop clipboard. To slide the snipping applet back away simply press ctrl+shift+alt again. Once you have done this, using ctrl+v will paste into the windows within the RDP session. The opposite is also true, for copying from the remote system to the local, using ctrl+c on the remote system to copy, then ctrl+alt+shift to slide out the snipping applet will show the content copied which you can then select and copy to the local system.