Since the start of 2020, most private insurance companies are now requiring that for patients to receive reimbursement for out-of-network care, their healthcare provider's NPI number and Tax ID must be registered with the insurance company.

Registering is typically a simple process. You can reach out to "Provider Services"  for the insurance company in question. 

Most of the BCBS plans have an online form.  For instance BCBS of Texas you can click here and it takes you directly to the form (be sure to select the "Participate out-of-network" option on the second step).