Thankyou very much Thondorin.
Yes the the custom control is a container ,
but they have disabled the numeric up and down buttons in the control
and yes we enter the value to the Textbox inside the custom control right.
I tried the below and I got NULL so I tried the second suggestion given by you
UIItem numpulint = (UIItem)window.Get(SearchCriteria.ByAutomationId("numPulseInterval"));
TextBox box = numpulint.Get<Textbox>(SearchCriteria.ByAutomationID("aWGNumericTextBox1");
Code that worked is given below
AutomationElement element = numpulint.GetElement(SearchCriteria.ByAutomationID("aWGNumericTextBox1");
TextBox box = new TextBox(element, numpulint.ActionListener);
from box I could read the value using box.Text