You can define a parameter as an ObjectScript expression that it is evaluated at runtime. To do so, specify its type as COSEXPRESSION and specify an ObjectScript expression as the value:

Parameter PARAMNAME As COSEXPRESSION = "ObjectScriptExpression";

where PARAMNAME is the parameter being defined and ObjectScriptExpression is the ObjectScript content that is evaluated at runtime.
An example class parameter definition would be:

Parameter DateParam As COSEXPRESSION = "$H";

Documentation.

That said, I'd recommend gradual refactoring of these parameters into methods.

If it's a part of Ensemble Production, you need to create Business Operation. Here's a sample BO that does POST request:

/// This operation does a POST request to a REST API and receives Auth token
Class Production.Operation.NLPAuthOperation Extends Ens.BusinessOperation
{

Parameter ADAPTER = "EnsLib.HTTP.OutboundAdapter";

Property Adapter As EnsLib.HTTP.OutboundAdapter;

Parameter INVOCATION = "Queue";

/// Get Auth token
Method GetAuth(request As Ens.Request, Output response As Ens.StringResponse) As %Status
{
    #dim sc As %Statis = $$$OK
    
    // Form request body (using Credentials)
    set input = {"user": ( ..Adapter.%CredentialsObj.Username), "pass": (..Adapter.%CredentialsObj.Password)}
    
    // Send post request
    set sc = ..Adapter.Post(.httpResponse,,input.%ToJSON())
    quit:$$$ISERR(sc) sc
    
    // Get token from response
    set token = {}.%FromJSON(httpResponse.Data).token

    //
    set response = ##class(Ens.StringResponse).%New(token)
    quit sc
}

XData MessageMap
{
<MapItems>
    <MapItem MessageType="Ens.Request">
        <Method>GetAuth</Method>
    </MapItem>
</MapItems>
}

}

If you're outside of Ensemble, you need to use %Net.HttpRequest class. Here's an example.

Let's say you have called this url:

http://localhost:57772/rest/users/1?fields=list

And you have this route:

<Route Url="/users/:id" Method="GET" Call="Test"/>

Then in your Test method call:

write %request.Get("fields")

it would output list.