To enter a waypoint using coordinates with ForeFlight Web, enter it in the format: 4015N10658W
Currently, 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. 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.