You use the PMT function in Microsoft Excel.
Note that there is a negative sign in front of the PMT function. This is to make the result to be a positive number.
The PMT Function Arguments dialog box shown below:
You need to divide the Interest Rate (B20) by 12 to calculate for monthly savings.
You also need to multiply the No of Payments, Nper (B19) by 12 to calculate for monthly savings.
The Future Value, Fv (B18) is the target amount you want to achieve. $1 Million is your target.
The Present Value, Pv (-B21) is the initial amount you start during the account opening. Then, you start saving every month. There is a negative sign because you are paying out of your pocket.
The Type is 0 as you start saving only 1 month after the account opening.