How to add custom waypoints to a FMC route ?

Hi all,

I loaded to the fmc a route to KIAD and noticed there is an almost 180 deg turn just 10nm before landing. I'd like to add a custom waypoint so to somehow "round" the path to the runway. In the PMDG 737 FMC I'd add a FIX/BRG/DIST waypoint like MOSBY/290/10, but I do not find a way to go with the 777 FMC.

Just opened the FIX page, added the FIX as per the FMC screen fields - MOSBY at LSK 1L, 290/10 at LSK 2L - and it looks there are no errors, but then I do not find ways to copy the fix, paste it into the legs page and include it into the active route I'm flying right now

Can someone advise on how to do ?

Thanks a lot in advance

Aldo