Emails with too vague requests are more likely to be ignored. In Germany, admissions to Master programs are usually handled centrally and specific professors do not have influence on your particular application. For doctoral student admissions, it is the opposite: you normally contact the professor directly and they decide whether they want to take you. In this case, having done some (good) research work at the department will increase your chances.
If your Master program includes a mandatory or optional research internship (which is not uncommon), applying for such an internship at the professor's department is (in my experience) one of the best ways to get in touch with them. The same goes for a Master's Thesis, though that will of course happen later.
If such an internship is not a standard thing at your university, you can always take initiative and ask someone at the department about your options to do research in the group. I would suggest to do this once you are there, so that meetings in person are an option. Professors are known to be busy, so it might be a good idea to contact one of the doctoral students/teaching assistants for this. If it is someone that you already know from a course, even better.
In summary, wait until you are there, and try to include specific requests in your email to increase your chances of success.
