Mine is this one:
Welcome to Worldbuilding, (Insert username)! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun!
The tour, help center, and Meta would all convert from magic links. The raw script (what you should copy+paste) is
Welcome to Worldbuilding, (Insert username)! If you have a moment, please take the [tour] and visit the [help] to learn more about the site. You may also find [meta] and [The Sandbox](https://worldbuilding.meta.stackexchange.com/questions/6168/sandbox-for-proposed-questions) useful. [Here](https://worldbuilding.meta.stackexchange.com/q/6388) is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun!
I'll modify as needed by changing parts of it or adding another notice. Remember to update it with the new link in case there's a new Sandbox (the current one is this one).
I adapted it from this answer from Are there any rules or a specific format for "Welcome to WorldBuilding!" comments? But felt that meta and The Sandbox should be mentioned, and later on the post on "Welcome to Worldbuilding!". Sometimes I'll customize it with magic links like
[chat], or things like "Nice first question!" If I mean it. I pop into the first Q/A review queue, add this message to each new post, and then hit the skip button — it lets me greet new users while letting others do the main review work, unless I feel like it that day.
Don't forget to change the "(insert username)" to the name of the user, or simply delete it. I've had it happen to me a couple times that I'll comment using this script but leave the "insert username" in and have to go back and fix it.