I noticed that some games in Roblox have a friend-request-alike prompt GUI. Is there any articles I should look into to find out how to recreate this in my own? 
Something like the badge/friend request prompt. ^
If this is the wrong section, please redirect me to the right one!
No, you’re good. This is the right section. You can use StarterGui::SetCore.
You can then pass for the first argument "SendNotification" since you want to well, notify a player.
Then you pass a dictionary.
For instance
local StarterGui = game:GetService("StarterGui")
StarterGui:SetCore("SendNotification", {
Title = "Hello!",
Text = "How are you doing?",
Duration = 3
})
Title is of course the title of the notification, whereas Text is the main contents (the body) of the notification. Duration is how long it should last (in seconds)
You can customize it even further; all of the documentation is in the API reference page linked.
Oh! Thank you! I was quite unaware of this page! Also, thank you for letting me know this is the wrong section. I marked your answer with a check! 