如何将其转换为自定义excel函数

我有以下c#代码，它从数据库中提取数据&在Excel加载时用数据填充单元格A1。我如何把它变成一个自定义函数，据此字段将填充当用户键入公式(即'=getMyData("mycustominput")')，而不是当excel加载?

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Xml.Linq;
using Excel = Microsoft.Office.Interop.Excel;
using Office = Microsoft.Office.Core;
using Microsoft.Office.Tools.Excel;
using MySql.Data.MySqlClient; 
namespace custom_excel
{
public partial class ThisAddIn
{
    private void ThisAddIn_Startup(object sender, System.EventArgs e)
    {
        Excel.Worksheet activeWorksheet = ((Excel.Worksheet)Application.ActiveSheet);
        Excel.Range firstRow = activeWorksheet.get_Range("A1", missing);
        firstRow.EntireRow.Insert(Excel.XlInsertShiftDirection.xlShiftDown, System.Type.Missing);
        Excel.Range newFirstRow = activeWorksheet.get_Range("A1", missing);
        string connString = "Server=localhost;Port=3306;Database=test;Uid=name;password=password";
        MySqlConnection conn = new MySqlConnection(connString);
        MySqlCommand command = conn.CreateCommand();
        command.CommandText = "SELECT field_value FROM customers LIMIT 1";
        try
        {
            conn.Open();
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
        }
        MySqlDataReader reader = command.ExecuteReader();
        while (reader.Read())
        {
             newFirstRow.Value2 = reader["field_value"].ToString();
        }
    }
    private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
    {
    }
    private void InternalStartup()
    {
        this.Startup += new System.EventHandler(ThisAddIn_Startup);
        this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
    }
}

}