How to add button to webform to set value to a field
You will have to create a button in a

function hook_form_FORM_ID_alter(&amp;$form, &amp;$form_state, $form_id) {
    // Your code here.
}


and use the #ajax property of the button to update the field when the button is clicked.

The examples D7 module has some nice AJAX examples that will show you how the mechanism works.