Try this: If you want to make a custom wpt xxxxN/xxxxE. Don’t just type xxxxN/xxxxE into the search bar, it won’t work. Instead, put it together with your fixes in the search bar.
For example if your fpl consist of these fixes “EGLL RWY27L NESKA” and you want 0200N/5000E to be the next wpt, you have to put “EGLL RWY27L NESKA 0200N/5000E” into the search bar. It’ll be troublesome if you’ve many wpts in your fpl as you’ll have to type every single fix. One thing good about this is that you can navigate your FPL all the way into the arriving runway of your destination airport should it not have arrival fixes.