尝试在FormView中设置TextBox1的默认值

protected void FormView1_DataBound(object sender, EventArgs e)
{
    TextBox textBox1 = (TextBox)FormView1.FindControl("TextBox1");
    textBox1.Text = "默认值";
}

protected void SqlDataSource1_Selecting(object sender, SqlDataSourceSelectingEventArgs e)
{
    e.Command.Parameters["@TextBox1"].Value = "默认值";
}

<asp:TextBox ID="TextBox1" runat="server" Text="默认值"></asp:TextBox>