How to point to custom extension / extensions_override_freepbx.conf

I need a multi conference number.
For that, i have made this addition to extensions_override_freepbx.conf:
exten => 709,1, Wait(1)
same => n,MeetMe(,cMdr) ; NOTE: If you add the option ‘e’, * will choose room # for you
;Change the ‘d’ option to ‘D’ if you want to have a pin number for the conference
; c gives the caller the number of participants already in the conference.
; r records every meeting in a folder (where?) by the date.

But now i have trouble how to find out where in the webUI i can make that extension(709)

Thankyou in advance.

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