Append meta field value in BuddyPress activity,

custom.js

// Add code in your theme's js file.
$.ajaxPrefilter( function( options, originalOptions, jqXHR ) {
	try {
		if ( originalOptions.data == null || typeof ( originalOptions.data ) == 'undefined' || typeof ( originalOptions.data.action ) == 'undefined' ) {
			return true;
		}
	} catch ( e ) {
		return true;
	}
	if ( 'post_update' == originalOptions.data.action ) {
		// Set your meta field id [ TEXT BOX - ID ].
		var activity_meta = '#activity_firstname';

		options.data += "&activity_meta=" + jQuery( activity_meta ).val();
	}
} );

functions.php

// Add code in your theme's functions.php file.
function bp_append_meta_text( $activity ) {

	$updated_content = $activity->content;

	if ( isset( $_POST['activity_meta'] ) && '' != $_POST['activity_meta'] ) {
		$first = '<div class="activity_meta"> ' . $_POST['activity_meta'] . ' </div>';

		$activity->content = $first . $updated_content;
		$activity->type = 'rtmedia_update';
	}

}

add_action( 'bp_activity_before_save', 'bp_append_meta_text', 10, 1 );