target="_blank" rel="noopener noreferrer"

But please read carefully.

Recommendations

Add rel=”noopener” or rel=”noreferrer” to each of the links that Lighthouse has identified in your report. In general, when you use target=”_blank”, always add rel=”noopener” or rel=”noreferrer”.

Or : means – nofollow will be enough to add.

btw: you can reach this via child-theme functions.php
adjust the webers-testseite.de through your home domain url:

add_action('wp_footer', 'open_external_links_in_newtab');
function open_external_links_in_newtab(){
?>
<script type="text/javascript">
(function($) {
	$('a').not('a[href*="webers-testseite.de"]').attr({
		'target':'_blank',
		'rel': 'nofollow',
	})
})(jQuery);
</script>
<?php
}

means that any link that does not match your domain url will be considered external and will be opened in a new tab with the attribute nofollow.

ok – take this code to child-theme functions.php:

add_action('wp_footer', 'open_external_links_in_newtab');
function open_external_links_in_newtab(){
?>
<script type="text/javascript">
(function($) {
	$('a').not('a[href*="webers-testseite.de"]').attr({
		'target':'_blank',
		'rel': 'nofollow noreferrer',
	})
})(jQuery);
</script>
<?php
}

but read on your link above : Collateral damage and https versus http

so it might be better to only do this to links which starts with http only:

add_action('wp_footer', 'open_external_links_in_newtab');
function open_external_links_in_newtab(){
?>
<script type="text/javascript">
(function($) {
	$('a[href^="http:"]').not('a[href*="webers-testseite.de"]').attr({
		'target':'_blank',
		'rel': 'nofollow noreferrer',
	})
})(jQuery);
</script>
<?php
}