Create foreign key from Entity in Another ERD

hi Every Body
I have Entity RelationShip Diagram.i create some entity in it.i want to create relation ship and my problem is that the entity is in another Entity RelationShip Diagram.what can i Do?

Please reference to the following movie about how to create relationship on model elements between diagrams. Feel free to contact me for any questions and wish you have a good day!

Best regards,
Rain Wong