[custom plugin]Can't get pagename

Good day dear WordPress community,

I hope that this is the right category, since I mostly read about existing plugins having troubles working. However, I am creating my own plugin with above basic php knowledge.

I am attempting to get the pagename, so that I can edit the page’s content when the url says /producten/ (Dutch for products)

However, I have 2 problems

The first being: the code I looked up on the internet, stated as resolved, doesn’t work. Here’s my piece of code to check if the code actually works:

global $post;
$pagename = get_post( $post )->post_name;
?>
<script type="text/javascript">
	<?php echo 'alert("Paginanaam: ' . $pagename . '")';?>
</script>

Here’s what will pop up: imgur

This also brings me to the second problem: I want the content to change as soon as the pagename reaches state if ($pagename == "producten") However, I do not know which hook to execute my function for the page to change content:

if ($pagename == "producten")
{
	add_action("wp_header","verander_titel");
	function verander_titel()
	{
		?>
		<div class="wrap">
			<h3>It worked!</h3>
		</div>
		<?php
	}
	verander_titel();
}