Populating value in a field of IDOC segment

Hello,

Need your help to understand the IDoc concept and issues I am facing

I have some information with me -

I have message type DELINS, and a field USR01 of segment E1EDK09.

Want to populate value in USR01, based on some logic.

This is outbound IDOC.

Could you please help me to know

1) What program/transaction will trigger this IDoc to send order to vendor?

2) Where should I write logic to populate values in field USR01 of segment E1EDK09 in DELINS?

Thanks & Regards,

Hemant