Here's how I'd write it:
"Dear [use name, rather than 'Madam', if known],
I understand that, in order to apply for my visa to study in Germany, I'm required to provide an admission letter which mentions the date when the summer semester will commence and which also shows the details of my enrolment.
I would be grateful if you could provide me with such a letter, in order that I can complete my visa application.
I offer my thanks in anticipation of your kind assistance in this matter"
HOWEVER . . . I think that the reference to 1 April 2021 in the existing letter provides the date that you're seeking anyway, as that's the official start of the summer semester at the University of Siegen:
https://www.uni-siegen.de/zsb/termine.html.en?lang=en
So it seems somewhat pointless to me to ask about it!