I don’t know how to put a placeholder [closed]

Use this code in your functions.php

function wpb_change_title_text( $title ){
    $screen = get_current_screen();
    if  ( 'post' == $screen->post_type ) {
      $title="Post Title";
    }
return $title;
}
add_filter( 'enter_title_here', 'wpb_change_title_text' );

The above code will Place “Post Title” as placeholder text in dashboard -> posts -> Add New. Replace that text with your text and also Replace post with your Post-type.

For more detail read this article