Display thank you message after form submit, no redirect

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HTML

PHP

Hello. I'm using Salesforce marketing cloud web collect form.

I would like a thank you message to be displayed instead of a redirect. I an't figure out how.

Please help.

What I have tried:

<?php
    $msg = $_GET['msg'];
        if ($msg == "err")
            { ?> 

<div>

<form action=" http://cl.exct.net/subscribe.aspx" method="post" name="subscribeForm" onSubmit="return formCheck(this);">
		<input type=hidden name="MID" value="uniqueID">

<input type=hidden name="lid" value="uniqueID">
  
<input name="thx" type="hidden" value="#" />

<input name="err" type="hidden" value="#" /> 

    <input type="text" name="Email Address" placeholder="Business Email Address"
style="padding:10px; min-width:250;">
    <br 

/><p></p>
    
<input type="hidden" name="subAction" value="sub_add_update" />

<input type="submit" value="SUBMIT" style="height:36px; width:100px; background:#286a92;font-
weight: 600; color:#ffffff; text-

transform: capitalize; border: 1px solid; text-transform:
capitalize;">

<script type="text/javascript"> 
<!--
var d = new Date();
var curr_date = d.getDate();
var curr_month = d.getMonth();
curr_month++;
var curr_year = d.getFullYear();
document.getElementsByName('SubscriberDate')[0].value = ("0" + curr_month).slice(-2) + "/" + ("0" + curr_date).slice(-2) + "/" + 

curr_year;
 
 function formCheck(formobj){
		
		 
	// Enter name of mandatory fields
	var fieldRequired = Array("Email Address");
	// Enter field description to appear in the dialog box
	var fieldDescription = Array("Email Address");
	// dialog message
	var alertMsg = "Please complete the following fields:\n";
	
	var l_Msg = alertMsg.length;
	
	for (var i = 0; i < fieldRequired.length; i++){
		var obj = formobj.elements[fieldRequired[i]];
		if (obj){
			switch(obj.type){
			case "select-one":
				if (obj.selectedIndex == -1 || obj.options[obj.selectedIndex].text == ""){
					alertMsg += " - " + fieldDescription[i] + "\n";
				}
				break;
			case "select-multiple":
				if (obj.selectedIndex == -1){
					alertMsg += " - " + fieldDescription[i] + "\n";
				}
				break;
			case "text":
			case "textarea":
				if (obj.value == "" || obj.value == null){
					alertMsg += " - " + fieldDescription[i] + "\n";
				}
				break;
			default:
			}
			if (obj.type == undefined){
				var blnchecked = false;
				for (var j = 0; j < obj.length; j++){
					if (obj[j].checked){
						blnchecked = true;
					}
				}
				if (!blnchecked){
					alertMsg += " - " + fieldDescription[i] + "\n";
				}
			}
		}
	}
 
	if (alertMsg.length == l_Msg){
		return true;
	}else{
		alert(alertMsg);
		return false;
	}
}
// -->
</script></form>
</div>
<?php 
}
?>