Triggers EVENT when the player enters the rectangle created by the 4 bounds.
To figure out where the TOP BOTTOM LEFT RIGHT coordinates in your level should be, open up the SMBX debugger in the level editor and note how the X and Y change based on your mouse. Move your mouse to figure out the coordinates for your trigger zone.
This command supports the "once" OPTION, meaning if you enter "once" as the final parameter it will only activate once and then delete itself.
// Activate event 1000 once when the player enters a certain area in the level TriggerZone,1000,-200576,-200384,-201344,-201244,once