Backup using adb on a phone with a dead screen
I was in the same situation as you.
Once you enter the full backup screen
Use the following commands:

adb shell input text &lt;your password&gt;
adb shell input keyevent 22
adb shell input keyevent 23


the "adb shell input keyevent 22" command is Dpad Right, you'll be able to select back up my data
the "adb shell input keyevent 23" is the Dpad Center, It will allow you to select enter

22 -->  "KEYCODE_DPAD_RIGHT"

23 -->  "KEYCODE_DPAD_CENTER"