c# ebay sdk ComboBox链接选定的交换事件按钮点击

我正在从按钮单击事件和从组合框索引更改事件选择类别的web请求。如何将在按钮单击事件中发出的请求链接到从组合框选定索引更改事件中做出的类别选择?

private void button3_Click_1(object sender, EventArgs e)
{
            CustomFindingService service = new CustomFindingService();
            service.Url = "https://svcs.ebay.com/services/search/FindingService/v1?";

            FindCompletedItemsRequest request = new      FindCompletedItemsRequest();
    request.categoryId = selectedItem.ToString();                
}
private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
    if (selectedItem.ToString() == "Cell Phones & Accessories")
    {
        this.comboBox2.Text = "";
        comboBox2.Items.Clear();
        this.comboBox2.Items.AddRange(catCellPhones2);
        selectedItem = "15032";               
    }
}