You can define association 1 to many and defining the relationship with both code snippet below. One is defined with a string, the other one with a property.
HasRequired with association defined by string:
this.HasRequired(a => a.Property1) .WithMany() .Map(a => a.MapKey("MyFK"));
HasRequired with association defined by property:
this.HasRequired(a => a.Property1) .WithMany() .HasForeignKey(a => a.MyFK);
Both mappings will create exactly the same database schema with a non nullable foreign key.
The mapping with MapKey is used when you don’t want to have the foreign key as a property in your model context class. This is called Independent Association.
The mapping with HasForeignKey when the foreign key is a property in the model. This type is called Foreign Key Association.
You can use the one you want. Personally, I prefer to use the HasForeignKey because it’s strongly mapped and if the property name is refactored that I am sure that the property will follow.