After that in PWM under the SMS Settings for 'SMS Gateway' you would have the following
SMS Gateway: https://api.clickatell.com/http/sendmsg
SMS Gateway User: the id you created for the API in clickatell
SMS Gateway Password: the password for api user above
HTTP(S) Method: GET
SMS Auth Method: Auth will be part of request
SMS Request Data: user=%USER%&password=%PASS%&api_id=YOURAPID&to=%TO%&text=%MESSAGE%
The rest is of the settings should be pretty easy. You can also use POST, you just need to configure it a little differently