Open iframe link after post PHP + JS

Question

I asked another question here on the forum about another subject that helped me a lot about this but the link opens in new page.

What I would like to do is to click on the Ubmit to open inside an iframe inside the page and if the field was not filled in receive a message (popup) stating that the field was not filled in..

<?php
if (isset($_REQUEST['txt_url'])) {
$link = 'http://' . $_REQUEST['txt_url'] . '.datatix.com.br';
header('Location: ' . $link);
} 
?>

<div class="instancia">
<div class="content">
<img src="img/logo_datatix.jpg" height="32px">
<form action="" method="post">
<input type="text" name="txt_url" value="" placeholder="Digite sua Instância">
<input type="submit" value="Ok" name="ok "class="ok">
</form>
</div>
</div>

<div class="iframe">
<iframe id="conteudo_iframe"></iframe>
</div>

If possible not to use php would be much better.

It would be more or less what is in the code below, but the user does not put the whole domain only the subdomain.

Example: It puts "test" and "OK" but in iframe it would load the link test.dominio.com.br

body{ padding:0; margin:0;}
.instancia{ background-color:#D3172F; color:#FFF; font-family:Segoe, "Segoe UI", "DejaVu Sans", "Trebuchet MS", Verdana, sans-serif; width:100%; position:fixed; border-bottom:#9B1022 2px solid !important;}
.instancia .content{ width:320px; margin:0 auto; display:table;}
.instancia img{ float: left;}
.instancia input{ height:30px; padding-left:5px; padding-right:5px; width:176px; border:0; float: left;}
.instancia .ok{ border:0; background-color:#9B1022; height:32px; width:32px; color:#FFF; float:left; font-weight:bold; cursor:pointer;}

<div class="instancia">
<div class="content">
<img src="img/logo.jpg" height="32px">
<form action="" method="post">
<input type="text" name="txt_url" value="" placeholder="Digite sua Instância">
<input type="submit" value="Ok" name="ok "class="ok">
</form>
</div>


</div>

<div class="iframe">
<iframe name="exemplo" width="100%"></iframe>
</div>

Answer 1

Use the attribute target of link or of form, with a value equal to that of the attribute name of <iframe>

<a href="/" target="exemplo">pt.stackoverflow.com</a>
<form action="/" target="exemplo">
  <input type="submit"/>
</form>
<iframe name="exemplo"></iframe>

Remark: the <iframe> will not finish loading due to OS restrictions, but saving the content in an HTML file will work.

Answer 2

Use the tag targert to identify that you want to open inside the iframe, if you want to do some other validation create a function in the onsubmit