Substitution OBBH during CO FI real time integration

Hello expert,

We have a requirement to post a value in the trading partner field during the CO FI real time integration (when using KSU5).

I've defined a substitution rule, the issue is that the program is not calling my substitution rule when creating the FI document.

I have tested my substitution with FB50 and it's working.

So do you have an idea why when using KSU5 I'm not able to use the substitution rule?

Regards,

Amine.