The register is part of a law that the Bundestag passed in 2020 following an initiative by a group of MPs led by today's Foreign Minister Annalena Baerbock. It will go into operation gradually: From today on, you should be able to register there using an ID card with an online function.
The register was actually supposed to start on March 1, 2022, but there were delays due, among other things, to the Corona crisis. It makes it easier for doctors to quickly and reliably clarify whether they are willing to donate.