If i enter Id in textbox1 then name should fill in textbox2

Question

For example: table name as custome

r 1.Customerid

 2.customerName

 3.Role customer

 wil regisert using this fields. 

Next They need to fill another table name as apply

 1.customerid(Here if customer enter him/her id it should go database)

 2.customerName(here name should get fill)

 3.product..

How to do this..