How to make an AppleScript app open another one? The title says it all. What would be the line of code to open a specific app created in AppleScript?

tell application "userappname" to open/launch/what?


What would be the last piece of the code? Many thanks in advance.
Let's say you have application XYZ.app and you want to open that from AppleScript, this would be the code:

tell application "XYZ"
    activate
end tell