When I put in or geolocate my location on the directions tab, if I click on a location on the map, after putting the curser on the end location it defaults that location to the start location.
We had holidays for 'HOLI' festival so we couldn't get back to you and feel sorry for your inconvenience. In direction tab when you click on a any marker then it will add in your start location automatically. Then you need to insert only End location manually.
Please share your page url where map is located so i can check it also at my end.