Copy GR quantity to characteristic in MIGO

Dear experts,

I have the following issue.

I have a class Z_BATCH and an assigned characteristic Z_QUAN.

What I want to do, is to automatically fill the characteristic Z_QUAN according to the quantity that is entered during MIGO.

I tried to define the refereence field in transation code CT04 using GOITEM-ERFMG or MSEG-ERFMG but without success.

So my question is, how can I automatically fill the quantity into the characteristic?

Thank you!