Derive a Key Field in SAP MDGF 7

Dear SAP MDG Guru,

I am implementing SAP MDG 7, and I have a requirement to derive a key field/ automatically assign value base on another key field value,

i already try to use BRF+ and Badi USMD_SERVICE but all of those will be trigger after the key field are fill, not when the key field are not fill,

Are there another solution for this? can some one help me with this case?

thanks before