If you need the empty span in the anchor you could just add that with jQuery. You can’t add it to a submit form button any way else. If you were hosting externally you would need to come up with a third party form so you would just edit the submit button as needed.
On webflow hosting, you could use this jQuery <script>$("<span></span>").appendTo(".btn-6-orange ");</script> to append the span to any element with the class of “btn-6-orange”. This code would have to be placed in the page or site “before body close” custom code area.
If you are going to try to style elements on the page with custom code, place that CSS in the " Inside Head" custom code area.