How to sign in with Guest Key

If you are referring to the guest key, check this out:
support.xbox.com/.../forgot-guest-key

A forgotten guest key can be reset by any adult as long as that person knows the password that they use to sign in to their Microsoft account on the console. The adult must be signed in on the console to reset the guest key.

I've already been to that site. What I understood is if no one is signed in, my guest can sign in with the guest key to play my games. Am I wrong?

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One adult has to have an microsoft account and sign into xbox. Then you add your kids under your account. However it was just easier for me to create an email account for them in yahoo or gmail. At first i hated this idea because previously the accounts
don’t allow minors to have emails so i had to lie about their ages. But then i warmed up to the idea because i realized that when they become of age their names they will be used up as the population grows, do i secured those email names but kept the passwords
to myself. Then i created individual accounts for them but kept the microsoft passwords to myself. I created 6 digit passwords for them which uses only xbox controller like XXXXXX, etc. in the end this is the easier route plus my kids game progress r saved
in the cloud and they can play on their grandpa’s xbox too

I want to let my guest sign in to play games without anyone else having to sign in first. I've already created a guest key. But go to the select a profile page I don't see an option to sign in as a guest. Am I missing something?