How to write PMT formula in c#

Sorry I did not seen this before but however I'm going to answer you :)

The =Pmt function returns the payment amount for a loan based on an interest rate and a constant payment schedule.

This is the Syntax:

Pmt( interest_rate, number_payments, PV, FV, Type )

interest_rate is the interest rate for the loan.

number_payments is the number of payments for the loan.

PV is the present value or principal of the loan.

FV is optional. It is the future value or the loan amount outstanding after all payments have been made. If this parameter is omitted, the Pmt function assumes a FV value of 0.

Type is optional. It indicates when the payments are due.
1 means payment are due at the beginning of each period (each month, for example)
0 means payment at the end of each period.

Let's see an eg:

This example returns the monthly payment on a $5,000 loan at an annual rate of 7.5%. The loan is paid off in 2 years (That is: 2 x 12). All payments are made at the beginning of the period.

=Pmt(7.5%/12, 2*12, 5000, 0, 1)

The 1 at the end means the installment is paid at the start of every month.

In Plain English:
7.5 (interest rate) divided 12 (monthly interest rate) for 2 years (2*12 months) for a loan of 5000, each payment is made at the start of the period.

sorry I don't know the syntax of PMT :$