To use your PayPal balance, select your PayPal balance as your “Preferred when paying online”. Your PayPal Balance should be the same currency as the one from the website (USD) and be enough to cover the whole amount.
Also, remember that you will still be required to select a credit or debit card as a backup because Placeit will request for you to open a billing agreement and avoid having to enter your PayPal details again. If there is enough PayPal balance for future subscription charges, the transaction will be completed using PayPal balance only.