How to retrieve values from Postman Request in C# code?

 
I have created a custom button "Custom Create Payment". This custom button triggers another standard Acumatica button "Create Payment" which opens a popup window. I want to use Postman Request to change the data in the popup window. Here is the postman request 
 

Web service endpoints:

public class SOOrderEntryExt : PXGraphExtension<SOOrderEntry>
{  
    public static bool IsActive() => true;       

    public PXAction<SOOrder> customCreatePayment;

    [PXButton]
    [PXUIField(DisplayName = "Custom Create Payment", MapEnableRights = PXCacheRights.Select, MapViewRights = PXCacheRights.Select)]
    public virtual void CustomCreatePayment()
    {
        using (new PXTransactionScope())
        {             
            var GraphExt = Base.GetExtension<CreatePaymentExt>();
            try
            {
                GraphExt.createDocumentPayment.Press();
            }               
            catch (PXException ex)
            {

            }

            var values = GraphExt.QuickPayment.Current;           

            values.CuryOrigDocAmt = 500;
            values.OrigDocAmt = 500;
            var Paymentgraph = GraphExt.CreatePayment(values, Base.Document.Current, ARPaymentType.Payment);
            Paymentgraph.SelectTimeStamp();
            Paymentgraph.Save.Press();               
        }
    }

}		

Now everything works as it should, but the data is substituted from the code. In this case, 500. I want to substitute the value that I specify in the Postman Request. How to catch this data in the code?