You need to specify where you want to send your request. Something like this should work.

Method OnD03Alert(req As User.Alert, Output resp As Ens.StreamContainer) As %Status
{
    #Dim sc As %Status = $$$OK      
    #Dim test As %Integer = 0
    
    Set httprequest = ##class(%Net.HttpRequest).%New()   
    Set httprequest.Server =  "www.usamobility.net"
    Set httprequest.Location = "cgi-bin/wwwpage.exe"
    
    Do httprequest.SetParam("PIN", ..PIN)
    Do httprequest.SetParam("MSSG", "motogplay")
    
    Set sc = httprequest.Post(, test, $$$NO)
    
    If $$$ISOK(sc) {
        Set stream = httprequest.HttpResponse.Data
        Set resp = ##class(Ens.StreamContainer).%New(stream)
    }
    Quit sc
}

Also Ensemble operations must return persistent objects so I replaced string with Ens.StreamResponse.

Some thoughts.

1. Do not use Hungarian notation.

2. Do not use ByRef params/dynamic objects/$lists/args... as an arguments. There are two exceptions:  you don't know the number of arguments or there's more than 10 arguments.

3. Method should not parse arguments, only validate arguments. If caller passed invalid input it's caller problem. Fail fast.

4. Mandatory arguments go first.

5. If several methods have similar signatures, the order of arguments should be the same.

6. Same logical arguments in different methods should have the same names.

7. Arguments should be consistent. If you pass primitives, pass primitives everywhere. If you pass dynamic objects, pass them everywhere.

8. Return types should also be consistent.

9. If it's object methods and you pass the same optional arguments in several methods extract these arguments as a property.
 

You can do it with a special property class. Here's how it works:

Class Testing.PropertyClass
{
Parameter myPropName;
}

Class Testing.MyPersistentClass Extends %RegisteredObject  [ PropertyClass = Testing.PropertyClass]
{
Property p1 As %String(MAXLEN = 5, myPropName = "myPropValue");
}

But I recommend you to check RESTForms project (part1, part2), which sounds like what you're doing -automatic REST interface for persistent  classes.

It also defines property class for similar purposes.