This link takes you to the account verification form.
When the channel reaches 100 thousand subscribers it gains the right to verify your account (a tick that is on the right side of the name of your channel).
So when you reach 100 thousand subscribers before asking for the account verification badge make sure that your channel has an art, a profile image, and a good description, because in addition to the 100 thousand subscribers, other things that are taken in consideration are these.
The request is reviewed by YouTube employees and can take up to a week to complete.
To access the YouTube verification badge request form click on the link below