User Exit to Override ASN IDOC field

Hello Experts,
I am looking for an user exit to override ASN IDOC field while sending IDOC to third party tool. Below is the scenario
Once we do the PGI on outbound delivery SPED output creates ASN and it triggers ZASN output. We use basic type DELVRY03 to send IDOC to third party tool. Now before sending ASN to third party tool we would want to send them a flag so based on that they perform auto receipt process in their system.
I have selected Export Indicator (EXPKZ) field from IDOC since this hasn't been mapped with third party tool. Once we get an user exit to update the Auto Receipt flag in the Export Indicator field in IDOC we will map it in XI/PI so that it will go to third party tool for further processing.
I have a logic to identify the ASN which needs to set auto flag but I am struggling where and how to put that logic, what user exit suffice the requirement?
Thanks in Advance


If my understanding correct Before you send inbound delivery information to third party system you will send one Flag to Third party system to do goods receipt.I have one question if system failed to create /user cancelled Outbound delivery then how will you stop auto goods receipt in Third party system ?.Also After goods receipt GR messages failed in sap as there is no ASN created in system.
If you want to send the flag before ASN creation then at Speed message level you split the Idoc message into two parts one for flag the message two third party system and another one to create Inbound delivery.