Hi Priya,
A question, how are you getting the POs which got their delivery number updated ? If you have some filtering criteria for them, then create report program, get them and ask the basis to schedule your report as a background task on say daily basis( or whatever is the need) at some time.
Coming to IDOC triggering part is it a Standard/Custom IDOC?Also is it Outbound/Inbound ?
For custom outbound you need to use a FM 'MASETR_IDOC_DISTRIBUTE'.
Please revert in case of further concerns.
BR.