Download extension from below url and export u will get button qliksense. There u can find for june 2017 version or previus version options.
Qlik Branch or GitHub - erikwett/qsVariable: Variable extension for Qlik Sense click on clone or download
Right Click on button and select properties.
Then in property window, select 'Actions' tab.. select 'Add'..select 'set Variable' and select your appropriate variable and mention the value to be assigned. I have highlighted the required tabs in yellow for your reference.
Hope it helps!