There is a filter: script_loader_tag. You can use it to find the correct script:
add_filter( 'script_loader_tag', function ( $tag, $handle ) {
if ( 'yourjsfilename.js' !== $handle )
return $tag;
return str_replace( ' src', ' defer="defer" src', $tag );
}, 10, 2 );
Please let me know if any.