bring up fbkeyboard with systemd

[SteveBetter]

I would like to bring up fbkeyboard with systemd on a phone device. So I created fbkeyboard.service as below:

[Unit]
Description=fbkeyboard

[Service]
Type=simple
ExecStart=/home/ubuntu/fbkeyboard_1 -d /dev/input/event2 -f /usr/share/fonts/truetype/noto/NotoSans-Bold.ttf
StandardInput=tty
StandardOutput=tty
TTYPath=/dev/tty1
Restart=on-failure
RestartSec=10
KillMode=process

[Install]
WantedBy=multi-user.target

The ubuntu login prompt appears, but no fbkeyboard.

When I modify the above [Install] section to
WantedBy=getty.target,
fbkeyboard appears without the login prompt and I can input characters on the blank screen.

What should I do to log in using fbkeyboard?

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I tried buffyboard with service file like the above. The system gets frozen and sometimes reboots itself.
fbkeyboard works fine on the device with the same kernel and a postmarketOS rootfs. The above service file is on a ubuntu rootfs.