You can pay for boosted posts from the Instagram app by adding a payment method. You can also pay for your boosted Instagram posts by linking a Facebook Page to your Instagram professional account and using your Meta ad account.
To add a payment method to your account:
Go to your profile and tap

in the upper right corner.
Tap
Settings.- Tap Business.
- Tap Ad Payments.
- Tap Payment Methods. From here, you can select a payment method that is linked from Facebook. You can also add another payment method, like PayPal, or a credit or debit card.
Note: If you change the primary payment method on your ad account, that change will be reflected across both Facebook and Instagram.