Back to previous form and setting control data

Dear all,
I have two web forms. One (Form1) with dropdownlist and button. Second (Form2) with the textbox , button and gridview.
I have the product list about 10000 items. When user go to Form1 and click the button. It will go to Form2 in which user will type the text in the textbox. And then click the button for filtering the productlist. the result will show in the gridview which will have select button in the first column.
When user select the product, Form2 will close and go back to Form1. The selected product will put in the dropdownlist in Form1.
 
How to do in the coding?
(1) Select button in the gridview with closing Form2, selected product put in the drop down list in Form1.
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