Get Notified if the Textbox is Empty

I would like to have your help regarding in Textbox 

like in the screenshot below, I want the user to have to enter Number of Quantity in one of the textbox to process the Add Button, how can I use if else to compare the one textbox is having a number and proceeds to Add item, Example, the Quantity the user puts 1 in CS so its now good to add item, but if the Quantity is empty the add item is disable to click or there is a notification. 

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use a notifier???

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i thing something like this. 

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If Textbox.Text = 0 OR is empty (Textbox.Text)

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How about a fast repeating Clock timer event with set btnAdd.enabled to is a number(textbox1.Text)  ?

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