How to define an experiment for first time users in Firebase?

I am trying to create an experiment using Firebase remote config. The criteria is as follows: It should target only new users who have not used the app ( opening the app for the first time)

Now on further research I found that there is a user property as below:

However, this is not available in the experiment window or a similar property that fulfils the above criteria in the Firebase console as seen below:

I can only see the user properties set by my code. One way I can think of is to use one of my custom user property which is not yet set (but set to a value like null), but I don't know how I can do this.

References

https://support.google.com/firebase/answer/6317486?hl=en https://firebase.googleblog.com/2016/10/better-user-targeting-with-firebase.html

Firebase has finally made it possible. https://firebase.google.com/docs/remote-config/config-analytics?authuser=0&hl=en#target_first_open