Sure, I can definitely help you with that! The formula you're looking for is called the Future Value of an Annuity with Compounding Interest formula. It looks like this:

Formula:

FV = P * [(1 + r)^n - 1] / r + PMT * [(1 + r)^n - 1] / r 


Whe

1. FV is the future value of the account
2. P is the principal (or starting amount) of the account
3. r is the interest rate per period (usually per year)
4. n is the number of periods (usually in years)
5. PMT is the annual addition to the account

Here's how to use the formula step-by-step:

1. Determine the values for P, r, n, and PMT. For example, let's say you have a starting principal of $10,000, an annual interest rate of 5%, a time horizon of 10 years, and you plan to add $1,000 to the account each year.
2. Plug those values into the formula:
   
   
   Formula:

   FV = $10,000 * [(1 + 0.05)^10 - 1] / 0.05 + $1,000 * [(1 + 0.05)^10 - 1] / 0.05 

   
   
3. Simplify the formula by calculating the values inside the brackets first:
   
   
   Formula:

   FV = $10,000 * [1.628895 - 1] / 0.05 + $1,000 * [1.628895 - 1] / 0.05 

   
   
4. Simplify further by calculating the values in the parentheses:
   
   
   Formula:

   FV = $10,000 * 32.57789 + $1,000 * 32.57789 

   
   
5. Add those two values together to get the future value of the account:
   
   
   Formula:

   FV = $10,000 * 32.57789 + $1,000 * 32.57789 = $48,778.90 


So, in this example, the future value of the account after 10 years with annual additions of $1,000 and an interest rate of 5% would be $48,778.90.

I hope that helps!