Alias E-Mail

I am trying to add an Alias e-mail to G-Mail.
I selected the address & it says to open the new address to get Validation code.
How can i open it ?
It was not Validated yet.
Any help appreciated.

Possibly useful links:

https://productforums.google.com/for...s/Mx3qHSm4DQAJ

https://www.wired.com/2016/03/set-gm...mport-inboxes/

http://www.techadvisory.org/2014/08/...lias-in-gmail/