Before you apply these macros you should take note that Klipper cuts against the grain when it comes to start and end code. In general, klipper likes to call a start and end macro and then the user can define what should happen within each macro within the klipper config. This de-embeds the start and end commands from the gcode and makes them flexible, even if you are printing the same file.
In principle this is useful but in reality it is not frequently used and therefore some people decide to stick with the start and end codes embedded into the gcode from the slicer. If you are one of those people then these macros will allow your klipper enabled BX to understand cura start code:
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# MARLIN TO KLIPPER MACROS
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