How do I configure Service Desk Plus to use the IntelliSoftware SMS Gateway?

Settings are as follows:

SMS Provider: Custom

HTTP Method: POST

https://www.intellisoftw.../smsgateway/sendmsg.aspx 

URL parameters:
Key - Value
username - Enter your username on the InteliSoftware service
password - Enter your password on the InteliSoftware service
to - ${{recipients}}
text - ${{message}}

Authentication details: None

Request Payload: Leave blank

Response from provider: OK

Send SMS message using Unicode characters for multi language: Uncheck