Of the fields target_amount, target_datetime and recurrent_amount, none are required. However if one is supplied, then a second must also be given. If two are supplied then the third parameter will be re-calculated in the response.
If the date the goal is created is 2023-01-01T00:00:00Z, if a savings goal is updated with the target_amount of £110, and the target_datetime is 2023-12-01T00:00:00Z, then the recurrent amount will be £10.
When performing this calculation, it is asusmed that a customer is not going to deposit their first recurrent amount within the month of creating the goal.
If this is the desired behaviour however, all three parameters can be supplied and the endpoint will accept it.