If you actually want to set the value in a field (not, e.g., set the value of a variable that you then use in set analysis), you should use the "Sheet Navigation and Actions Button" that is now just called "Button" in the standard set of visualizations (November 2020). In earlier versions you can find in the "Qlik Dashboard Bundle" set of extensions.

(worst case, it is still up in Github: https://github.com/stefanwalther/sense-navigation).

You'll need to create two buttons: on the first ("left") button you'll set the action as "Set values in field" where you choose "List" as the field" and 'X' as the value. On the second ("right") button you'll set the action to "Clear values in field", for the field "List". There are some options to style by expression. You can do something like, If(Only([Field]) = 'X', style1, style2) to dynamically style the button so that it looks more like a checkbox (e.g., use a dark background if X is selected on the left button, white if it is not selected, reverse for right button).

hope that helps

v