I

IDで指定

HtmlElement usernameButton = webBrowser1.Document.GetElementById("auc_shipname_standard1");

usernameButton.SetAttribute("value", "定形郵便");



//説明文入力時にＨＴＭＬの方で開始するためのＪＡＶＡＳＣＲＩＰＴを機動
object[] args = new object[] { "true" };
webBrowser1.Document.InvokeScript("ModeTabTEXTON", args);

HtmlElementCollection forms = webBrowser1.Document.All;

//タイトル
forms["Title"].SetAttribute("value", T_Title.Text);

//終了時間
if (ComEnd.Text.Trim() != string.Empty)
{
HtmlElement element2 = webBrowser1.Document.GetElementById("ClosingTime");
HtmlElement childElement2 = webBrowser1.Document.CreateElement("option");
childElement2.SetAttribute("id", "option-1");
childElement2.SetAttribute("value", ComEnd.Text.Trim());
childElement2.SetAttribute("selected", "true");
childElement2.InnerHtml = ComEnd.Text.Trim();
element2.AppendChild(childElement2);

}

//商品発送元の地域
if (ComTodou.Text == "北海道")
forms["loc_cd"].SetAttribute("value", "1");
else if (ComTodou.Text == "青森県")
forms["loc_cd"].SetAttribute("value", "2");

//出品者（落札者は無料）
if (Rhassou2.Checked)
{

forms.GetElementsByName("shipping")[0].SetAttribute("checked", "");
forms.GetElementsByName("shipping")[1].SetAttribute("checked", "true");
}
else if (Rhassou2.Checked)
{
forms.GetElementsByName("shipping")[0].SetAttribute("checked", "true");
forms.GetElementsByName("shipping")[1].SetAttribute("checked", "");

}

//Yahoo!かんたん決済
if (cKessai2.Checked) forms["ypmOK"].SetAttribute("checked", "true");
else forms["ypmOK"].SetAttribute("checked", "");

//銀行振込
if (cKessai3.Checked) forms["aspj1"].SetAttribute("checked", "true");
else forms["aspj1"].SetAttribute("checked", "");

//販売形式//////////////////////
if (Rhanbai.Checked)
forms.GetElementsByName("salesmode")[0].SetAttribute("checked", "true");
else if (Rhanbai1.Checked)
forms.GetElementsByName("salesmode")[1].SetAttribute("checked", "true");
else if (Rhanbai2.Checked)
forms.GetElementsByName("salesmode")[2].SetAttribute("checked", "true");

//返品の可否
if (R_henpin2.Checked)
{
forms.GetElementsByName("retpolicy")[0].SetAttribute("checked", "");
forms.GetElementsByName("retpolicy")[1].SetAttribute("checked", "true");
}
else if (R_henpin1.Checked)
{
forms.GetElementsByName("retpolicy")[0].SetAttribute("checked", "true");
forms.GetElementsByName("retpolicy")[1].SetAttribute("checked", "");

}
//入札者評価制限(総合評価で制限 )
if (ComSeigen0.Checked) forms["minBidRating"].SetAttribute("checked", "true");
else forms["minBidRating"].SetAttribute("checked", "");

〇フォームコントロールの指定で
HtmlElementCollection all = webBrowser1.Document.All;
HtmlElementCollection forms = all.GetElementsByName("q");
forms[0].InnerText = "c#"; // テキストボックスに「C#」を入力