Generate password using caddy hash-password:

caddy hash-password

Add to Caddy File a basic-auth snippet with the obtained hash:

(basic-auth) {
        basicauth /* {
                bob $2a$14$0Lh5......RfjeG
        }
}

Import to Caddy File the snippet where needed in site block:

example.com {
    import basic-auth
    root * /site
    file_server browse
}