OK found it my self !
To all who wants to try, it does work for me:
From the inibuild store download the iniManager and install.
Finish all your setting after opening the manager, simbrief with your username or ID, i use the username because it is asked within the FMC
In there go to Pilot support and there at pilot support export flightplan, should show your last created
The planfile gets DL and is stored in your Official/OneStore/microsoft-aircraft-a310-300\Data\FMS plans\
For example this is my Path:
d:\MS Flugsimulator 2020\Official\OneStore\microsoft-aircraft-a310-300\Data\FMS plans\
Go to your FMC and put the plan in there, for example:
Plan is safed as OERKOJAI.fpl so put in OERK/OJAI, that should do it..
Hope this helps and Happy Flying !
cheers