Within the Third Generation Editor (3GE) we do not have the ability to add a "View as webpage" link at the moment. We are collecting feedback from our customers around this feature and I have passed along that you would like to see this in accounts.
Thank you for pointing out this older request again! Our design team listened to the requests, and have since added that feature back, and given you more control than you had in the older editor. In any text field of our Third Generation editor, just click the 'Insert' button, and choose 'View as Webpage' from the drop-down menu.
This will let you insert a link that opens the web-page version of the email, and it also gives you all of the control you would normally have over text, so you can change the color, alignment, size, as well as being able to choose where in your email you want to put that link.