How find and fill unknown TextBox inside a form with another c# prog using with SPY++

I have a software installed on my personal computer (windows 7) and with another c# program (win-form , visual studio 2013) i want to access a text box inside that installed software and fill it.

so i used SPY++ to find that text box.

but SPY++ only recognize that form and show same Class for both that window and text box and when i want to choose that text box nothing happens and it seems i am still on that windows form instead of that text box.

so what is your suggestion to access that text box?

Here is the picture for both windows and textbox...