OK, two ways.
Generically, you can refer to a form’s parent like this :
self.parent.xxxxx
which will then allow you to access any component on the parent, including other forms.
Secondly, and probably better if you are always using the container form as the reference or top level form, is to use :
get_open_form()
Here’s @meredydd explaining its use :
but in brief, that function refers to the form that was last opened with open_form(), either explicitly called or implied by the default form for the project.