thanks for reaching out to the wefugees community.
The decision to donate an organ is a very personal one. It is documented either in the organ donor card, which you already have, or in your living will, which can be notarized. But you should take the costs into account. Whereas you there free downloads for a living will (e.g. here: https://www.chip.de/downloads/Patientenverfuegung-Vorlage-PDF_143846880.html),
a notarization has to be paid.
As far as blood donation is concerned, you are already registered with the organization where you donate blood. The reason that asylum seekers need a tax number is simply that every person who lives in Germany is subject to taxation, even new born babies.
I hope my answers were useful for you and I'll be very pleased if you contact me again with further questions. I'm a member of the MBEON-staff and I‘ll be glad to guide over a longer period of time. Mbeon is an online consultancy for migrants via the mbeon-app, which you can install on your smartphone for free. Download it via App Store or Google Play Store. You'll find me In the mbeon-app under my name „München, Frau Müller“ and get confidential one-to-one advice in a safe virtual room where nobody can wiretap messages. Further information: www.mbeon.de.