Hi Gopi,
You need to have a Text_ID to create / save TEXTS...
Via customizing you define which text id is allowed in a purchase order (and at which level)
If you open the text editor when you are in any text of your PO, then you can choose from menu of text editor GOTO > header and you will see which text id this text has.
You can use that Text ID in your FM which then saves to your Corresponding PO!
Hope this helps.
Cheers,
Kripa Rangachari.