I found it in Technical -> User-Defined Defaults 
I added record 

Field -> Responsible (mrp.production)
Default Value (JSON format) -> User ID
User -> User

And it works to new created MO by user, but if scheduler creates MO it still assigns system user (ID1)

Hello,

I'm not sure for Odoo 12, but in developper mode, when you click on the developper menu, you can Set Default values for these fields.

If the responsible is not always the same and based on specific rules, I'd recommend configuring automated actions.

Hope it helps.