Hello @Maxi-7240 , the sambaNTPassword attribute is a SAMBA exclusive attribute. Schema extensions aren't supported in Azure AD DS so you can't add any new. Provided you can store the value in a differente named attribute then you can leverage Custom attributes for Azure Active Directory Domain Services.
Let us know if you need additional assistance. If the answer was helpful, please accept it and rate it so that others facing a similar issue can easily find a solution.