How to add default content in post editor

Add the following code to your theme’s functions.php:

add_filter( 'default_content', 'my_editor_content' );
function my_editor_content( $content ) {
    $content = "REPLACE THIX TEXT";
    return $content;
}

Source: http://www.wpbeginner.com/wp-tutorials/how-to-add-default-content-in-your-wordpress-post-editor/