It's /E for the transponder type. For ADSB I use B2 but as you've found there seems to be no code for what many of us have: both 978 and 1090 ADSB-IN.
Edit: from the FAA's guide to ICAO flight plans:
B1 1090 MHz out capability, or
B2 1090 MHz out and in capability
U1 UAT out capability, or
U2 UAT out and in capability