create a multiple lines of text column in SharePoint list,specify the type of text to allow Enhanced rich text (Rich text with pictures, tables, and hyperlinks).
Then you can use the code below to save the content in InputFormTextBox into the SharePoint list field:
<SharePoint:InputFormTextBox runat="server" ID="inRichTextBox" Rows="10" RichText="true" TextMode="MultiLine" RichTextMode="FullHtml" AllowHyperlink="true" ></SharePoint:InputFormTextBox>
<asp:Button ID="Button1" runat="server" Text="Button" onclick="Button1_Click" />
2.The C# code:
protected void Button1_Click(object sender, EventArgs e)
{
SPSite site = SPContext.Current.Site;
SPWeb web = site.RootWeb;
web.AllowUnsafeUpdates = true;
SPList list = web.Lists["list"];
SPListItem item = list.GetItemById(1);
item["InPutFormField"] = inRichTextBox.Text;
item.Update();
web.AllowUnsafeUpdates = false;
}