SSH tunnel in .ssh/config with public key on second leg

I want to reach Ubuntu20.04 server U1 through Ubuntu20.04 server U2 since U1 is behind a firewall, from a MacBook with BugSur. SSH from U2 to U1 is done with a public key, without password. I get to U2 using password but the next step always asks for a password (which I can’t remember now). I have seen this question and tried things like this in the MacBook’s .ssh/config:

Host U2
    HostName accessible.uni.com
    User me
    # PreferredAuthentications publickey
    ForwardX11 yes

Host U1
    HostName behind_firewall.uni.com
    User me
    # PreferredAuthentications publickey
    # IdentityFile /home/me/.ssh/id_rsa.pub
    ProxyJump U2
    ForwardX11 yes

and running ssh U1 enter the password for U2 but is then asked for password for U1. How can I make it work password-free on the second ssh?

>Solution :

You can use Remote Command. Once it’s successfully connected to U2 it will execute the command ssh U1.

Host U1
    HostName accessible.uni.com
    User me
    # PreferredAuthentications publickey
    ForwardX11 yes
    RemoteCommand ssh U1