The dropdown list has an "onSelect" event to which you can associate a script in the screen editor. That script provides you with two extra objects.
- "source" is the screen element that triggered the event, the dropdown list object itself. This might be useful if you wanted to use one script to manage multiple dropdown list objects. If not, you can probably ignore this
- "newKey" is the value (value, not label) that was just selected
So, if you made the labels on your dropdown descriptions - "Create purchase order" - and the values transactions codes - "ME21N" - then this simple script will do what you are asking:
session.startTransaction(newKey);