Add new button is missing in sub grid dynamic 365 CRM

Question

Dear all I am new to Dynamic 365 cRM , I have a customer entity and customer form I have another entity customer_Order in which I have customer orders I want to show customer orders on customer forms how to achieve that .

 

Below is the detail what i did

 

I have created another entity Customer Order and in that entity I added 1-N relationship with customer table , i put a sub-grid I customize my form as well I did not add any field on customer entity nor i add any relationship there .

 

My sub grid is appearing but not showing add new button.