In the widget blueprint, simply click the ‘Graph’ button in the top right. This switches to the actual logic editor. From there, look to the left side of the window to the ‘My Blueprint’ pane. Then click the ‘+’ symbol in the ‘Variables’ tab. From there just set the type to integer and give it a name.
Here is the button I am talking about. Pardon the text, a computer mouse is not the best writing implement.
EDIT: You will also need to click the eye if you want to access the variable from other blueprints.