Add client

I am adding new clients to a database using ASP.net I want to check if the username the client has chosen is not already in use by another client and highlight the relevant input box and write a message to select a different user name if it is. Is there way to write to a specific place in the HTML similar to innerHTM? I haven't been able to find such. Alternatively i guess I could try an interact with client side Java, or re-write the whole page. Thanks, Steve