https://travel.state.gov/content/travel/en/us-visas/Supplements/Supplements_by_Post/FRN-Frankfurt.html
As of May 2024, the U.S Embassies Zagreb, Croatia; Bratislava, Slovakia; Ljubljana Slovenia; Nicosia, Cyprus; and the U.S. Consulate General Amsterdam, the Netherlands will cease routine immigrant visa (IV) services. The U.S. Consulate General Frankfurt has been designated to conduct IV services for these posts in the following categories: immediate relative, family preference, employment based, fiancée/fiancé (K), follow-to-join asylees/refugees (V92/V93), and diversity visas (DV). For more information, visit the following website.
NVC probably hasn't updated its form letter yet, but it looks like you will do the process in Frankfurt.