From HMRC:
Tax credits customers to receive Cost of Living Payments letter
HMRC has just sent letters to tax credits customers with information on the first Cost of Living Payment.
It is important to note that receiving a letter doesn’t mean that a customer is eligible for the payment. They must meet the eligibility criteria as outlined on GOV.UK.
Customers do not need to contact HMRC or apply – payment is automatic into the bank account where they receive their tax credits payments. If they do contact HMRC, they will be unable to provide more information as everything the customer needs will be on GOV.UK and in the letter they receive from HMRC.