https://www.united.com/web/en-US/content/mileageplus/default.aspx -login to your account then go here.
Register for a new Thanks Again account(you need a credit card), then login and go to “my account” and click where it says “Click Here to Join Thanks Again Offers”.
Then you should see a link that says, “Enroll Today and Earn 1000 Bonus Miles”. Click that, and sign up with your Visa card. It takes 4-6 weeks for the miles to show up in your Thanks again account, then you can transfer them to United.