The EVN segment always has the same format:

EVN|O01

All you need to do is set the constant field using the outbound script for Event Type Code like you did for the MSH constants.

To add the code to the EVN segment's outbound script:

• Under Segments, double-click EVN (Event Type). The EVN Segment window appears.

• In the Outbound Script column, click the ellipsis button for Event Type Code. The Editing Script window appears.
• Type the following code in the script window:

value="001" 

• Click OK to save the change.
• Click the Transform button. You will notice the change in the bottom pane of the Transformation window in real-time.