To use your PayPal balance, just choose it as your “Preferred when paying online” option. Make sure your PayPal balance is in the same currency as the website (USD) and has enough funds to cover the full amount.
Also, keep in mind that you'll still need to select a credit or debit card as a backup. This is because Placeit requires you to set up a billing agreement, so you don’t have to enter your PayPal details every time. If your PayPal balance is sufficient for future subscription payments, the charges will be made using your PayPal balance only.