How to set up BizTalk WCF-WebHttp Send Port for POST method

Question

I am working with a REST API within BizTalk Server 2013 and I need to make a POST to send an SMS using WCF-WebHttp Send Port. Content-Type is application/json. I have so far put in the Address (URI) something like the following:

https://sendsms.com?

and in the HTTP Method and URL Mapping section I have something like:

<BtsHttpUrlMapping>
   <Operation Method="POST" Url="mobile={mobile}&amp;text={text}"/>
</BtsHttpUrlMapping>

I need to retrieve these 2 parameters from a SQL table (table simply contains mobile column and text column). This is where I'm unsure about what needs to be done as I've seen different ways of making POST method calls.

Could I create a receive port that polls the database to retrieve the row and promote these as context properties for the variable mapping?

Or do I need to create a Http Request Body schema to generate a request xml object?