Step 1: Click on the button called "Place Bid" to enter the Bid Form.
Step 2: In the form under Bid Detail, there is an empty field [1] where you can enter your bid Alternates. If you want to add more then one bid Alternate, you can click on the link [2] "+ Add alternative bid" to add more then one if needed.
NOTE: You can edit your bid as many times as you want before the bid deadline.