Promotional credits are coupons Meta gives to encourage advertising. You cannot buy these officially from Meta. :
If you want to "buy" balance for your ads ahead of time to strictly control your budget, you can add prepaid funds: buy facebook ad credits
: Sign up for services with official partners like Shopify , GoDaddy, or Wix, which often offer ad credits as a signup perk. Promotional credits are coupons Meta gives to encourage