functions.php

add_action( 'wpbf_content_close', 'wpbf_add_beaver_builder_to_content_close' );
function wpbf_add_beaver_builder_to_content_close() {
    // Replace id= with your actual Beaver Builder saved row or template ID
	if ( is_singular( 'post' ) ) {
    	echo do_shortcode( '[fl_builder_insert_layout id=506]' );
	}
}