Show page text on 'post page'

I found 2 topics which have the same question.
https://www.ads-software.com/support/topic/adding-static-text-to-top-of-posts-page?replies=4
https://www.ads-software.com/support/topic/add-static-text-to-the-blog-page?replies=5

The first link does describe, but now how to do this more dynamic, like with a custom field.

Second link no answers to lack of response by the topic starter.

Create a template file for the page.

do you have a link for your site?

This is the code used for my index.php
I use index.php because, if i want more blog pages i want them to be exact the same.

https://jsfiddle.net/Aftn3/1/

What i want is some static text from the page editor or a custom field appear above the loop.

Layout:
Some description about the blog posts (static).

loop:
post1
post2
post3
etc.

My website:
https://www.bewustsite.nl/

if you see my home, you see 2 custom fields one at the top with some heading text and one at the bottom with heading text. Between there is some paragraph text.
I want the blog page exactly like this, so that the paragraph text is replaced by the loop

I hope todo this like i do on page.php

<?php echo get_post_meta($post->ID, "header", true); ?>

<?php if (have_posts()) :  while (have_posts()) : the_post(); ?>
<h4><?php the_title(); ?></h4>
<?php the_content(); ?>

<?php endif; ?>

anyone can help me out?