Inserting Javascript yields 403 error

According to previous posts in this forum, it should be possible to insert custom Javascript code into the or fields of the default theme, but whenever I try to do so, I get a popup saying “403 error”. Given that I am the admin but not a developer, how can I add my one line of custom Javascript?

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You may have no permission to using the scripts resources by some Referrer-Policy, make sure it’s public access or others right permission.