Hi @Simi
What you need is an official notarisation, in German called ‘amtliche Beglaubigung’. You can do this either at a notary or your local citizens’ office ‘Bürgeramt’. Doing it at a notary generally costs more.
To have it done at your local Bürgeramt, you have to show them that the notarisation is required by another German authority, in this case the confirmation from the embassy.
You bring the confirmation from the embassy, your ID and the letter of consent. You sign the letter of consent in the presence of an official and they stamp it to certify that it was you who provided the signature.
Here is the information from the website of Niedersachsen about how to do it:
https://service-niedersachsen-de.translate.goog/detail?pstId=8664322&_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp
The embassy requires the original of the notarised letter of consent.
Best,
Éanna