Many users ask that they want to send categories as #hashtags instead of separated by vertical bar.
If you want the same, you can add this code at the end of functions.php of your active theme.
Note: If there is ?> at the end, then place the code before that

add_filter( 'wptelegram_post_categories', 'wptelegram_post_categories', 10, 1 );
function wptelegram_post_categories( $string ) {
 if ( ! empty( $string ) ) {
  $string = '#' . str_replace( '|', ' #', $string );
 }
 return $string;
}