You must be logged in and on the search results screen before you can place a hold.
Click on the checkbox next to the item:
Click on Place Book on Hold:
Enter your email address in the box field if you would like to be notified when the item becomes available. Then click on Submit:
If you do not enter an email address you will be prompted to confirm. Click OK to continue or Cancel to go back:
You will receive a confirmation that your items was placed on hold: