Let us divide this into two parts the server side and client side on server side we have to write a function or page method.
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Function itlsef is pretty simple all it does takes two strings join them and return the result. Thats all we need to write for server side.
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Client side has its own requirements first its better to use post request if you want any result back from server and then you should use json as communication medium and for that we need to set content type and data type as
Last requirement is your web.config should have entries for asp.net ajax sections. If you are building from scrath you can shoose asp.net ajax website as template when starting new website.
Please read articles from following links to know more and have a strong background.