What file to edit?

I would like to inject the following code into my pbx:

exten => s,1,AbsoluteTimeout(30)

Now my question is, which file do I modify and how do I go about doing so?

I get the following comments when I attempt to edit my extensions.conf file. Thanks

; Do NOT edit this file as it is auto-generated by FreePBX. All modifications to ;
; this file must be done via the web gui. There are alternative files to make ;
; custom modifications, details at: http://freepbx.org/configuration_files ;

Try custom contexts in FreePBX

There may be a general setting in FreePBX that you can change to do this rather than adding a custom context.

where exactly do you want to add that?