To enter a waypoint using coordinates with ForeFlight on the web, you can either enter it in Degree Minute format: 4015N10658W, where Latitudes must be 4 digits and Longitudes must be 5 digits using the Degree Minute format (if the Longitude is less than 100 then enter a leading zero eg: 4030N08630W)
Image 1. Degree Minute Coordinate Format on ForeFlight Web
or you can enter a waypoint in Decimal degree format: 32.70454/-116.64010 32.70788/-116.71793 32.70288/-116.71793 32.70088/-116.64010
Image 2. Decimal degree Coordinate Format on ForeFlight Web
Additionally, if you already have a route planned, you can 'rubber band' to a specific lat/long by clicking on a point on a route leg and then dragging it to the desired lat/long location.
Also if you locate the position on the map, you can right click on the map and select the lat/long value for that location to add it to your route.