I want to insert a new entity and set its FK to the related master entity, which already exists.

I don't know the PK value of the master entity,but I know a Unique value of the master entity.

I tried this unsuccessfully

           var detailEntity= new detailEntity();

            detailEntity.Popis = "Default";
            detailEntity.masterEntity= new masterEntity{UniqueField= "XXX",IsNew = false};

and

           var masterEntity = new masterEntity();
            masterEntity.UniqueField= "XXX";

           masterEntity.IsNew = false;

            masterEntity.detailEntity.Add(new detailEntity{ Popis = "Default" });

The PK value gets set by a sequence.

I know a Unique value of the master entity.

If you don't have the PK value of the master entity, then you have to fetch it first, before you associate the detail entity to it. And since you know the Unique value, you can fetch it using that value.