Update field VIQMEL-REFNUM in tx IW21

Dear experts,

I need to enter a value in field VIQMEL-REFNUM during the creation of a PM Notification (IW21),

I could able the field VIQMEL-REFNUM using the Screen type hdr = H700, but I just can display the field, I´m not able to enter values.

I have tried to modify the field in OIAL, but I couldn´t.

Please, somebody knows how can i enter values in field VIQMEL-REFNUM ?

Thank you!

Regards,

Viru.