Your use of the_content
filter is wrong. Please check how it should be properly used in the codex in the link supplied
First of all you are totally replacing the_content
with nothing, that is why you get a blank screen. The reason is that you are not calling the $content
variable in your function
Secondly, if you’ve called the variable, you would have replaced all content with Test text here as you are overwriting all the content, and not appending it
This should do the job
add_filter('the_content','post_add_me');
function post_add_me($content){
$text="Test text here";
$content = $content . $text;
return $content;
}