Quick and simple method if your just using 2 different portals
Player On Collision with (Portal) > Player set position portal2.X/Y
You can stylise it a bit better by putting your movement controls into a group and disabling the group, add a fade and do a simple:
Player On Collision with (Portal) >
Set group (controls) deactivated, start fade (your fading object)
On fade in finished > player set position portal2.X/Y
On fade out finished > set group (controls) activated