[Gammix]

First you need to detect if the player is near the desired location, use this under a callback like OnPlayerPosChange (not a SAMP function but can be found here: https://sampforum.blast.hk/showthread.php?tid=573961)
https://sampwiki.blast.hk/wiki/IsPlayerInRangeOfPoint

Now when the player is near, use this:
https://sampwiki.blast.hk/wiki/Function:SetPlayerCheckpoint

To make a dialog open up whenever the player enters the CP, use this callback:
https://sampwiki.blast.hk/wiki/OnPlayerEnterCheckpoint

Or Streamer plugin may help you: https://sampforum.blast.hk/showthread.php?tid=102865