This should work everywhere. If not, the internet is broken, the world is coming to an end, find a cave and stay there
String js = 'document.querySelector("[name=my_input]").value = "";'
WebUI.executeJavaScript(js, null)
If the input has an id attribute:
String js = 'document.querySelector("#my_input").value = "";'
WebUI.executeJavaScript(js, null)
Good luck. (but you shouldn’t need it)