You can do this in a custom mapping in the immunization channel. Example channel below

Steps to add.
1. Click insert on the #4 mapping node or add a new node to the editor.
2. Check the "Customize Script" button.
3. Copy the following code into the script window:
var rxrArray = message.getAllNodes('RXR');
for (var i=0 ; i < rxrArray.length ; i++) {
var rxrSeg = qie.parseHL7String(rxrArray[i]);
if (rxrSeg.getNode('RXR-1.1') == 'PO') {
message.setNode('RXR-2', '', (i+1));
}
}
4. Enter a description for the mapping "Update RXR-2 when RXR-1.1 equals 'PO'"
5. Click ok and save and restart the channel