You have two different script triggers that you can consider. The simplest to use is OnObjectExit. This can perform a script to take you to the next record and even to a specific field in that next record. But any action, such as clicking with the mouse, that exits the field will trip this trigger and produce this result.
Instead, you can use the OnObjectKeystroke trigger if your script checks the code for the specific keystroke and does this action only if the key pressed was the tab key.
If [ Code ( Get ( TriggerKeystroke ) ) = 9 // code for a tab key ]
Go to Record/Request/Page [Next ]
Go to Field [ Specify a field here] // or use go to object if you give the desired object and object name
Exit Script [ False ]
The Exit Script [False] step cancels out the tab character so that it does not get processed by FileMaker after the script completes.