Replace the whole Masdar Muawwal, not only أَنْ.
● أُرِيدُ أَنْ أَخْرُجَ فِي سَبِيلِ اللَّهِ — I want to go forth in Allah’s path.
● أُرِيدُ الْخُرُوجَ فِي سَبِيلِ اللَّهِ — I want to go forth in Allah’s path.
The complete أَنْ أَخْرُجَ is replaced by the explicit verbal noun الْخُرُوجَ.