Pop up window and send variable to current page

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Hello guys!

 

What if i have a form for adding new product. One of the fields of the TableProducts is 'CategoryId'. So the most common way to enter the CategoryId is by a dropdownbox. What if the category does not exist yet? So you have to add the new category from another page and refresh the "add product" form to load the newly inserted category. Can anyone guide me how to make a pop up page where i can add the category and upon submitting the new category, the categoryid of the "add product" form will have the new inserted categoryid. 

 

Thank you so much guys.