The EF Core template generates fluent mapping for all classes and its properties (including foreign keys) in ModelName.cs.
If you want to add custom attributes:
1) double click diagram surface to open Model Settings and navigate to Model > Settings > Attributes > select the assembly (e.g.: System.ComponentModel.DataAnnotations.dll) with the needed attributes and make sure that the needed attributes (e.g.: ForeignKeyAttribute) are checked in the window below, press OK
2) select the needed attribute in the Attributes collection of a particular class property