Keeping TO header when dialing from Asterisk

Hi everyone,
I have a question about configuring the extensions.conf

I want every incoming call to be directed to one extension, that would process the call and then eventually call the desired number.

Let’s say there are two extensions within Asterisk - 100 and 101 and that there is an incoming call to extension 100. What I want is Asterisk to call extension 101, but keep the To header field as 100, so that the processing unit at extension 101 knows that call was intended for extension 100. (It doesn’t need to be To header, but I need some field to indicate who the call was intended to).

Could you please give me a hint on how to accomplish this?

Much thanks for any advice

Ok, nevermind…

I have resolved it by using SIPAddHeader()