How to Add text to the start of all comment?

You can use this filter hook to add this

apply_filters( 'pre_comment_content', string $comment_content );

Here is the code how you can use this

function add_custom_comment_content( $comment_content ) {

    // You can handle it many ways 
    $comment_content .= "Custom Content"
}
add_filter( 'pre_comment_content', 'add_custom_comment_content', 10, 1 );