foreign key

Question

I have a problem understanding how the foreign key concept works, i know the theory but the practice cause me problems. For eg I have the table 

client(client_id(pk),first_name,last_name, user_id(fk)) and 

user(user_id(pk),cient_id(fk),password,e-mail)

and i want to make a form that adds a new client. When adding in client table an user_id, in user table the user_id is filled automatically? or should i add it?  

hope it makes sense what i've explained.