C# - Auto populate the field of Currency Type if the Code is entered

Question

How should I add the code to the application's script that is already there? After entering the specific code, the currency field must automatically populate. I'm new to C#, so I have no idea how to add the code.

The code that is currently being used on the application is shown below.

//Parameters:
//DataPool data
//ITrace trace

//pick the first four digits from barcode number as company code;

foreach (Document doc in data.RootNode.Documents)

{
//definition of fields
 Field Bcode = (Field)doc.Fields["BarCode"];
 Field Coco = (Field)doc.Fields["CompanyCode"];
 Field Dpay = (Field)doc.Fields["ZDC_DOWNPAYMENT"];
 Field Vendmt = (Field)doc.Fields["ZDC_VEND_MAINT"];
 
  
 //if there is no barcode or improper barcode, assign "0000" as default barcode value, 
 //which is used as default company code;
 
//Coco.Value = Bcode.Value.Substring(0,4); - seem to cause stuck at ocr when placed here
//Coco.State = DataState.Ok;

   Coco.Value = "----";
   Coco.State = DataState.Ok;

 if (Bcode.Value.Length < 5)

{
   //Bcode.Value = "00000" ;
   Bcode.Value = "" ;
   //Bcode.Value = "10100" ;
   //Coco.Value = "0000";
   //Coco.State = DataState.Ok;

}

 else
{
    //Coco.Value = Bcode.Value.Substring(0,4);
    //Coco.State = DataState.Ok;
}

//Coco.Value = Bcode.Value.Substring(0,4);
//Coco.State = DataState.Ok;

If I entered this code 1010, 1020, 1030 like shown below photo.

The currency must auto populate into USD not manually entered.