We need an input field widget with "send" button. This widget will change an EmonCMS feed value.
You can fork the existing widget Curl ( [url removed, login to view] ) or Button ( [url removed, login to view] ) from EmonCMS dashboard
Settings for this widget is max value, mix value and feed to modify. When the widget first load, it is set to the value of the feed. After feed update by the "send" button, it will update it value in the input field.