HDL for Terminating Worker, including Action Reason

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I need to use HDL to end work relationships for a batch of employees. I managed to do this successfully but had not included the Action Reason. In this case, it's the end of fixed term appointments. When I view Action Reasons in the UI, I can see 'End of Fixed Term Appointment' with code 'HES_END_FTA'. I've tried using both (name or code) in the Worker.dat upload but keep getting the error 'The value of the attribute action reason code isn't valid'. Can anyone advise me where I'm going wrong/ how I can find a valid Action Reason for inclusion