MyStringExtension.cs

using System.Text;

public class MyStringExtension {
    
    public string InjectComma(string input)
    {
        StringBuilder sb = new StringBuilder(input.Length * 2);
        for (int i = 0; i < input.Length; i++) {
            sb.Append(input[i]);
            if (i % 5 == 4) {
                sb.Append(",");
            }
        }

        return sb.ToString();
    }
}