This plugin creates a discord bot which would be hosted on the server itself which lets you use the following commands through Discord's slash commands:

• whitelist add {player}
• whitelist remove {player}

This is the config file (on display to view what customisability the plugin offers)

# Discord Bot Token - Get this from https://discord.com/developers/applications
token: YOUR_DISCORD_BOT_TOKEN_HERE
allowedDiscordRoles: []

# Command templates executed by the server console.
# Use {player} as a placeholder for the Minecraft username.
whitelistAddCommand: "whitelist add {player}"
whitelistRemoveCommand: "whitelist remove {player}"

# Discord reply messages.
# Placeholders:
# - {player} / {username}: the Minecraft username
messages:
  noPermission: "You don't have permission to use this command!"
  errorNoSubcommand: "Error: No subcommand provided!"
  errorNoUsername: "Error: Username option not provided!"
  errorPluginNotInitialized: "Error: Plugin not initialized!"
  invalidUsername: "Error: Invalid username! Minecraft usernames must be 3-16 characters (letters, numbers, underscore only)."
  successAdded: "Added **{player}** to the whitelist!"
  successRemoved: "Removed **{player}** from the whitelist!"