Hi Cullen,
That field isn't encoded and probably should be, and would make sense in the UI, but, this works:
nrcq
http://nagioshost/rest modify/hosts \
-d name:diskstation \
-d 'notes_url:/highcharts/highcharts.html?host=$HOSTNAME$%26srv=$SERVICEDESC$'
If you want it added to the UI then please create a github issue - thanks!
Cheers!
Mark