It very simple:
The custom script contains a function like
function smshub_custom1 ($msg, $mobile, $flash, $sender, $username, $password, $account, $url, $proxy, &$error) {...}
This function is called by SMSHub to send the SMS. The call will receive all the parameters.
Ex: $mobile will contain the user mobile number and $msg will contain the SMS message to be sent.
Just drop all the content of the function and add your own PHP code to send the $msg to $mobile with your custom SMSC service.
If, like you say, it's a simple call to a URL with parameters, just use CURL PHP to make an HTTP GET request to the service.
Ex.
$result = curl_exec($ch);
curl_close($ch);
Note that some of the parameters are the SMSC settings which you configure in the SMSHub graphical configuration in WebADM.
Ex : password, username, sender, url...
So you can put your custom SMSC login password in the graphical config and use the parameter. Same for the URL.
Note that the custom function must return a "true" on success.
Be sure to read and follow the SMSHub README to correctly configure OpenOTP and SMSHub to work together.
OpenOTP calls SMSHub service to send the SMS. there is a login with a static username/password to be set in SMSHub configuration and in OpenOTP. Then if SMSHub is configured with a Custom SMSC, it will call your script.