Custom-fields in Activity-content

Hi, i need add custom-fields to activity-content.

I can’t do this. In this example I show that the text “this yes” appears in activity and “get_post_meta? function not appears. This function is what I need to show my custom fields.

add_filter( 'bp_blogs_activity_new_post_content', 'record_post_activity_content' );
function record_post_activity_content(){
return get_post_meta($post->ID, 'txt_link', true).'this yes';
}

I think the problem is that i can’t get the post id. I tried with this code and it works:

return get_post_meta(577, 'txt_link', true).'This1';

I tried in many ways but I can't get the post-id to work with bp_blogs_activity_new_post_content filter

Thanks