Submit form that posts to another site - asp.net

Question

I have an online banking login that I'm trying to take from an old client site and put on their new site. If I copy the form and paste it into my new site, it just reloads the page when submitted. I've tried to change it to an asp.net form using the appropriate and other applicable tags with the same result.
If I take the form and put it in a simple index.html file, it submits and takes me to the correct external site.
I was unable to set the ID & values with the asp.net form and that may be one of the issues.
Any help is appreciated.
Here is the HTML form version that works in .html page that needs to work in .aspx page:
<form action="some external site" method="POST" autocomplete="OFF" target="_top">
<input type="hidden" name="sssid" value="my value">
<input type="hidden" name="iid" value="another value">
<div class="field-container">
<label for="aid" id="aid-label">Access ID</label>
<input name="aid" id="aid" type="text" size="8" onFocus="toggle_label(this, 'focus');" onBlur="toggle_label(this,'blur');" />
</div>
<div class="field-container">
<label for="passcode" id="passcode-label">Passcode</label>
<input name="passcode" id="passcode" type="password" size="8" onFocus="toggle_label(this, 'focus');" onBlur="toggle_label(this,'blur');"/>
</div>
<div class="form-row">
<input type="submit" name="Submit" value="Go" class="button">
</div>
</form>
Here is my asp.net form version that I tried:
<form action="some external site" method="POST" autocomplete="OFF" target="_top">
<input type="hidden" name="sssid" value="some value">
<input type="hidden" name="iid" value="another value">
<asp:TextBox runat="server" ID="sssid" CssClass="form-control" type="hidden" ClientIDMode="Static"></asp:TextBox>
<asp:TextBox runat="server" ID="iid" CssClass="form-control" type="hidden" ClientIDMode="Static"></asp:TextBox>
<div class="form-group">
<asp:Label runat="server" id="lblAccessID"></asp:Label>
<asp:TextBox runat="server" ID="txtAccessID" CssClass="form-control"> </asp:TextBox>
</div>
<div class="form-group">
<asp:Label runat="server" id="lblPassCode"></asp:Label>
<asp:TextBox runat="server" ID="txtPasscode" CssClass="form-control" TextMode="Password"></asp:TextBox>
</div>
<button type="submit" class="btn btn-blue">GO
<img src="/Content/img/icon-sm-arrow.png" alt="" />
</button>
</form>
My asp.net form code behind:
protected void Page_Load(object sender, EventArgs e)
{
txtAccessID.Attributes.Add("placeholder", "Access ID");
txtPasscode.Attributes.Add("placeholder", "Passcode");
}

Answer 1

I had to use the following to post this correctly:
<asp:LinkButton runat="server" ID="btnSubmit" CssClass="btn btn-blue" PostBackUrl="urlhere" Text='GO <img src="/Content/img/icon-sm-arrow.png" alt="" />' />

Answer 2

set PostBackUrl property of Button control => https://msdn.microsoft.com/en-us/library/system.web.ui.webcontrols.button.postbackurl(v=vs.110).aspx
the reason webforms always posting it 2 itself may be due to their __doPostBack script.