In campaign design step page, you can click the button “Insert Special Link” and select the Forward/View Message/Unsubscribe link you want to insert.

Insert Special Link in design page with CK editor 

Insert Special Link in design page with drag and drop editor 

In html design page with drag and drop editor, you can drag and drop the text button into your design area, and then click  to extend text editor. You can click “Special links” and select Forward/View Message/Unsubscribe link you want to insert.