With this Update you can now specify commands that run when the player enters the main-portal or the target-portal.

If one command will teleport the player, the default teleport to the other world is denied.

You just need to add a line like in the following image and specify if it will be executed by the 'player' or 'server', followed by ':' and the command.

You can also use the <playername>-tag in the commands to insert the playername of the player who entered the portal.