YWLTB is a Telegram bot.
It can be found in Telegram at @YouTubeWatchLaterBot.
It does one simple thing: adds incoming YouTube videos into your "Watch Later" playlist.
But don't worry!
First, you don't need to provide this bot your username and password. It uses OAuth to access your YouTube account. OAuth uses short-lived tokens with a limited scope to access data via API. It means that your credentials are not exposed to the bot and the access is limited only to the YouTube API. The bot cannot access other resources like your Gmail or Google Calendar.
Second, you are in full control of this access. You can revoke it at any moment in your Google Account settings.
Third, this bot is an Open Source Software. You can review the codebase, hosted on GitLab and GitHub, for any potential threats, and decide to trust it or not. You can even deploy your private instance of this bot and use it. The instructions can be found in the README of the aforementioned repositories.