v2fd_1.cs

var parser = new PipeParser();

var oml = new OML_O21();

/* 
    MSH (Message Header)
    V282_CH02_Control 2.14.9 pg. 59 
    https://www.hl7.org/fhir/terminologies-v2.html
*/

// Required Fields

// Field 00007 - Date/Time of Message DTM
oml.MSH.DateTimeOfMessage.Value = DateTime.Now.ToString("yyyyMMddHHmmss");

// Field 00010 - Message Control ID ST
oml.MSH.MessageControlID.Value = Guid.NewGuid().ToString("N");

// Field 00011 - Processing ID PT (2.A.58)
//  C - Processing ID - https://www.hl7.org/fhir/v2/0103/index.html
//  C - Processing Mode - https://www.hl7.org/fhir/v2/0207/index.html

oml.MSH.ProcessingID.ProcessingID.Value = "D";
oml.MSH.ProcessingID.ProcessingMode.Value = "Not Present";

var pipe = parser.Encode(oml);

Console.WriteLine(pipe);