This simple code works for me:

    let your_instance="masto.ai";

    document.location=(
        "https://"
        + your_instance
        + "/authorize_interaction?uri="
        + encodeURIComponent(document.location)
    )

Before you turn it into a bookmarklet, set your_instance to the Mastodon instance you use.

You can use my bookmarklet editor to turn it into a bookmarklet:

https://www.gibney.org/bookmarklet_editor