Populate editor with some content of a page with a page template

The is_page_template() function will not work in the admin. The second parameter of the default_content filter is the $post object. Your best bet is to leverage that to get the stored value for the page template in the post meta information:

add_filter( 'default_content', 'wpse_editor_default_content', 10, 2 );

function wpse_editor_default_content( $content, $post ) {

    if ( 'page-salespage.php' === get_post_meta( $post->ID, '_wp_page_template', true ) ) {
        $content = "If you like this post, then please consider retweeting it or sharing it on Facebook.";
    }

    return $content;
}

Note that this will only work if you have already assigned the page template and saved it as a draft or published it, otherwise the meta value will not have been assigned yet.