How can I put link that automatically show under every posts.

what do you want it to do/say?

You can either:

Add the HTML code to your template file(s). This may be needed in more than one place depending on how complex your sites theme is.

Add a function to tack the HTML onto the back of any content. As a (not checked too well…) example:

add_filter('the_content' , 'my_link_function' , 100);

function my_link_function( $content ) {
    return $content.'<p><a href="https://site.com">My Link Here</a></p>';
}

if you add it to the theme’s template files make sure that you’re using a child theme!

Thanks for your kind replies.

I’ve actually prepared a form in a special page where I want the link to direct whoever clicks “click Here To fill Our form”

That should appear under every post, both old, new and yet to be posted.

I am using a child theme of MesoCulumn theme