Change your logging.json file to output debug logs for Jellyfin.Plugin.Webhook. Make sure to add a comma to the end of "System": "Warning"

{
    "Serilog": {
        "MinimumLevel": {
            "Default": "Information",
            "Override": {
                "Microsoft": "Warning",
                "System": "Warning",
+               "Jellyfin.Plugin.Webhook": "Debug"
            }
        }

Use Handlebars templating engine to format notifications however you wish.

See Templates for sample templates.

• if_equals
  • if first parameter equals second parameter case insensitive
• if_exist
  • if the value of the parameter is not null or empty
• link_to
  • wrap the $url and $text in an <a> tag

• Every Notifier

  • ServerId
  • ServerName
  • ServerVersion
    • $major.$minor.$build
  • ServerUrl
    • Server url
  • NotificationType
    • The NotificationType

• BaseItem:

  • Timestamp
    • Current server time local
  • UtcTimestamp
    • Current server time utc
  • Name
    • Item name
  • Overview
    • Item overview
  • Tagline
    • Item tagline
  • ItemId
    • Item id
  • ItemType
    • Item type
  • Year
    • Item production year
  • SeriesName
    • TV series name
  • SeasonNumber
    • Series number - direct format
  • SeasonNumber00
    • Series number - padded 00
  • SeasonNumber000
    • Series number - padded 000
  • EpisodeNumber
    • Episode number - direct format
  • EpisodeNumber00
    • Episode number - padded 00
  • EpisodeNumber000 -
    • Episode number - padded 000
  • Provider_{providerId_lowercase}
    • ProviderId is lowercase.
  • RunTimeTicks
    • The media runtime, in Ticks
  • RunTime
    • The media runtime, as hh:mm:ss

• Playback

  • Includes everything from BaseItem
  • PlaybackPositionTicks
    • The current playback position, in Ticks
  • PlaybackPosition
    • The current playback position, as hh:mm:ss
  • MediaSourceId
    • The media source id
  • IsPaused
    • If playback is paused
  • IsAutomated
    • If notification is automated, or user triggered
  • DeviceId
    • Playback device id
  • DeviceName
    • Playback device name
  • ClientName
    • Playback client name
  • NotificationUsername
    • User playing item. Note: multiple notifications will be sent if there are multiple users in a session
  • UserId
    • The user Id
  • PlayedToCompletion
    • true/false, Only when NotificationType == PlaybackStop

• Discord
  • MentionType
  • EmbedColor
  • AvatarUrl
  • BotUsername
• Gotify
  • Priority
• Pushbullet
• Pushover
  • Token
  • UserToken
  • Device
  • Title
  • MessageUrl
  • MessageUrlTitle
  • MessagePriority
  • NotificationSound
• SMTP
• Slack
  • BotUsername
  • SlackIconUrl

Future events can be created from https://github.com/jellyfin/jellyfin/blob/master/Jellyfin.Server.Implementations/Events/EventingServiceCollectionExtensions.cs