How to Override Frappe Core

I Have a Doctype “Client” when i fill out this Client doctype i want to create User from this Client doctype and how i can implement this in my custom App,
required field in "Client Doctype "

  • List item
    email
    client_name
    certified_since
    certified_expiry
    client_code
    region
    address

also added user child table has Role and also selected Client Role from here ,