Guess I'll start with the spiel...
Know that WRITE_SECURE_SETTINGS means that app can change almost everything in your settings app, possibly including other settings not listed there. I'd be very careful about the app you're granting access.
Anyway, this is supposedly the way you're supposed to do it, but last time I tried it didn't work. I think it's because I added access to logs to an app, and the grant access prompt didn't pop up for a secondary user. Guess that won't be an issue for you, but who knows. Let me know if it works!
adb shell pm list users
Get the user id you want, i.e. my "Facebook" user's id is 17:
adb shell pm grant --user USER_ID com.package.name android.permission.WRITE_SECURE_SETTINGS