The exam TestDaF is in the B2/C1 range and therefore requires an advanced language level. It is therefore recommended that you attend a C1 course first.
Not only for university applicants, but also for professionals it can be useful to take the TestDaF exam. The standardized exam is also a meaningful proof of language proficiency when looking for a job and increases your qualification.
Please note that the registration deadline for the exam is approximately one month before the exam.