The question is, why to you believe that proof of accomodation must be supplied at all (i. e. is mandatory) when making the application?
The application form for a D-Visa states that you you should give the address if known (for the C-Visa application, this not stated that clearly).
D-Visa Application form (December 2019) Question 6:
Vorgesehener Aufenthaltsort in der Bundesrepublik Deutschland Intended place of stay in Germany
- Straße, Hausnummer (sofern bekannt) Street, number (if known)
- Postleitzahl, Ort Postcode, place
Wie werden Sie untergebracht sein? How will you be accommodated?
- Einzelzimmer Single room
- Wohnung mit Zimmern Apartment with rooms
- Sammelunterkunft Collective accommodation
- Sonstiges (bitte erläutern): Other (please specify):
Both applications require to to confirm, in different ways, that the information is given to best of his or her knowledge and belief:
- C-Visa: I declare that to the best of my knowledge all particulars supplied by me are correct and complete.
- D-Visa: The applicant is obliged to provide all information to the best of his or her knowledge and belief.
The wording states clearly what you intend to look for, not what you have achived. If someone has done this for you in Germany, then you can give that address as proof.
So if you don't have that proof, leave it blank.
You can also address this in a short covering letter, making it clear what your intentions are - thus avoiding any misunderstandings.
No one expects, when making a application, that you have a legally binding contract for a long term accommodation, just as they don't require you to have a fully paid return ticket.
Looking for a shared flat has been extremely difficult online for me ... and I am quite sure that I will be able to find a shared accommodation by the time I reach there or book a hotel/hostel for a week and find a shared accomodation there itself in person.
This attitude is completely reasonable and realistic.
Upon arrival (where the conditions for entry will be checked), showing that you have an accommodation for the first week is, however, advised.