Issue using script to update dnsexit DDNS

Edit: Asus RT-AC66R, Merlin 378.55 firmware

I followed the instructions at https://github.com/RMerl/asuswrt-merlin/wiki/Custom-DDNS and created a script to update DNS with dnsexit. When I run the script from a shell I get:
HTTP/1.1 200 OK
10=Hostname is not specified

When I copy/paste from the script my version of
http://update.dnsexit.com/RemoteUpdate.sv?login=<username>&password=<password>&host=<domain>&myip=
into a browser I get:
HTTP/1.1 200 OK
0=Success

I...

Issue using script to update dnsexit DDNS