Webhook signing secret generated from Mux video service.
You must add a webhook to your Mux project, if you doesn't have one, just go to <a href="{muxLink}" target="_blank" rel="noopener noreferrer">this link</a> and create a new webhook.
Add URL <a href="{muxCallbackUrl}" target="_blank" rel="noopener noreferrer">{muxCallbackUrl}</a> to URL to notify field then save it.
After you got a webhook, copy Signing Secret and paste it to this setting.