It depends on what country you are in. For example, in the UK stuttering is protected characteristic. But typically in this context it usually means the employer is just making it clear they are happy to make accommodation for people who identify as having a disability or barrier. For example, you could just mention to HR that you have a stutter when invited to an interview. Or you might even mention it on your CV/resume. And this would trigger the HR team and interviewer to understand your situation better when assessing your application. You typically don't need a certificate for this. Just talk about it.