I have a Polygon system also and need to set the option 160 to https://dm.novumnetworks.com.au:443/dms/Polycom_VVX400/. Your command line seems similar to what I'll require. When I tried your first line though I got an error, "The specified configuration node is not valid. Set failed".
I use the CLI initially but am now using Putty so I can copy-paste in directly. Any idea why it's failing for me?
To clarify, the line that isn't working:
set service dhcp-server global-parameters option option-160 code 160 = string;