Unable to call JSON API using Webhook

Unable to call JSON API using Webhook

Hi,

I am trying to create a webhook which essentially calls a SMS JSON API. Here is the instruction from SMS service company.

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API URL: http://api.sendsms.pk/sendsms/b7fe073f06b3eb32d2ec.json  
Accepts three parameters by HTTP POST request 
phone = phone number to which the sms will be sent, 
msg = message, 
type = 0 for regular and 1 for flash sms
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I am simply not able to create a web hook which could successfully call this API. I have tried different variations in webhook add form. I am testing hook through a workflow. Can someone explain how to properly create a web hook meeting above API requirements?

Thank you.

Aziz