You are unable to customise the monthly repayment amount.
You can try to adjust the loan amount and the interest rates so as to achieve the closest monthly repayment amount.
The calculation is done on a yearly basis, and thus if you are keying it in on the 3rd month of a yearly repayment cycle, the repayment amount might varies from the actual amount.
Ultimately, it's meant to be give the closest projection, and over a period of 20 to 30 years, the difference is not significant to the charts.