Of the fields target_amount, target_datetime and recurrent_amount, only two are required. If two are supplied then the third parameter will be calculated in the response.
If the date the goal is created is 2023-01-01T00:00:00Z, if a savings goal is created 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 assumed that a customer is not going to deposit their first savings payment 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 this.