I have successfully passed data from Form to Form and subform to subform but cant find any information on how to pass from a form to a subform. Let me explain;

I have 2 forms, A and B. 

Form B is embedded into Form A as a subform. I want to achieve the following;

>> On Success of Form B >> Push values to Form A Subform.

I know how to fetch the IDs but I cannot work out the correct expressions to pass the values.