You need to be a developer of "Microsoft 365 and Copilot" apps. When adding yourself as developer or editing an existing developer (https://partner.microsoft.com/en-us/dashboard/account/v3/organization/identity?publisher=true&panelOpen=AddPublisher-SelectProgram) choose the program "Microsoft 365 and Copilot".

It may take some time until the Office tab (now it is "Microsoft 365 und Copilot") appears at the offers page (https://partner.microsoft.com/de-de/dashboard/marketplace-offers/overview).