How to activate Google Inbox from your computer?

Step #1: Get an invitation email from your friend

Step #2: Open the mail, come to the footer of the mail and find the following
"As long as this email is in your Gmail, you can download the app and activate Inbox by Gmail on that account. Not that you need it, but the invite code generated for you is:" 
Next to this line the Invite code will be there.

Step #3: Append the token to the following URL

https://mail.google.com/mail/btinvites/mobilesecondary?token=

If, it not works, try the following
https://mail.google.com/mail/btinvites/mobile?token=

A file will be downloaded (Nothing to worry).

Now, enjoy, Google Inbox without an android/Ios