Praise be to Allah.
You can pray it straight after praying ‘Isha’.
It says in Fataawa al-Lajnah al-Daa’imah (8/144):
It is prescribed for travellers and those who are sick to join prayers, and also for non-travellers to join prayers on a rainy night. They may then pray Witr immediately after praying ‘Isha’ when it is joined to Maghrib at the time of Maghrib.