I think you're doing the right thing because they need the FOI endorsement to be eligible for a flight instructor's certificate anyway, but you dont need the endorsement to go sit for the actual FOI written knowledge exam.
An interesting thing is that one can get a ground instructors certificate with no FOI endorsement (61.213)
But regardless of whether you already hold a ground instructor's certificate, or even if you are a professional teacher and are exempt from taking the FOI test, you still need to get the instruct'rs endorsement covering FOI knowledge areas before you can qualify to get a flight instructors certificate 61.183(d)
So, it's kind of a weird requirement for flight instructor applicants where the endorsement and the test are two separate requirements not dependent upon each other, but nothing prevents one from sitting for the FOI without an endorsement.
You can contrast it with requirements for other certificates, like the commercial written where an instructor must 'certify' that the applicant was trained and ready for the written exam 61.123(e)(2)