Using the intrinsic function Ref on a Stack created within another Stack only gives you the Id of the referenced Stack. If you want to get the StackName which is generated automatically you have to do a combination of the intrinsic function Split and Select as follows:

!Select [1, !Split ["/", !Ref MyStack]]

This works since the Stack Id is structured as follows:

arn:aws:cloudformation:eu-west-1:*********:stack/test-nested-MyStack-R5E52GRQGVZH/8d90dd40-17a7-11ea-b079-02c18823f600

The statement splits the Stack Id by deleimiter “/” resulting a list which contains the StackName on index 1 which we then can select using the Select function.