MRB engineers are certified through your employer under the authority of the customer. When starting out working in MRB departments you will complete repair schemes and stress analysis' which will be signed off by the lead MRB engineer until they think your competent enough.
Then you should apply to the customer, who's products you will be dispositioning, for signature authority.
The customer will give you delegated authority to sign off concessions on you own.
This process can take months if your just starting out as an MRB engineer. More experience will give you greater powers in the department.