Hi SaidurRahaman
If you did not register your phone in your Microsoft Account and if you did note deactivate the Reset Protection you cannot get the key to do it now!
You can try the solution presented by Leandro in a pots here https://www.youtube.com/watch?v=4RQtSq0oqDM
I have never tried it, I don-t know if that works, but you can try it and then give us a feedback