Change User Role On register

Hi , I just wanted to set a different User Role when someone register with a agent ID. How could I do this.

I don't know where to check the condition and apply the change. (Programmatically)

Some code like that  : 

on registration form or just after 

if(agent_id === ' empty '){

 user set_role(customer)

}else {

user set_role(client_vip)

}

Please help me out on this , it's the only things left for me to do.

Thank you so much , it working very well  , you resolve this so fast . 

Thanks again