User exit to default reason code on MI07

Hi MM Experts,

Am an ABAPer and need your expert advice on this. For some reason I have been asked to default the reason code on MI07 screen with a certain value  as soon as  the user gets on the screen. I have not been able to find a user exit where I can default the value on MI07 for reason code.

All MM experts can you please suggest a user exit where we can do this.

Regards,

Saket.