Usage of filter : bp_activity_new_update_content

bp_add_username_in_activity.php

// Use this filter in your theme's functions.php file if you want to filter the content of new activity.
// For example below code will append username to current activity item.

// define the bp_activity_new_update_content callback
function filter_bp_activity_new_update_content( $activity_content ) 
{
    // Append additional content here...
    $user_info = get_userdata(get_current_user_id());
    $activity_content = 'I am user : '.$user_info->user_login." ".$activity_content;
    // remove the filter
    remove_filter( 'bp_activity_new_update_content', 'filter_bp_activity_new_update_content', 10, 1 );
    return $activity_content;
}
        
// add the filter
add_filter( 'bp_activity_new_update_content', 'filter_bp_activity_new_update_content', 10, 1 );