This snippet is used in the following knowledge base article – https://kb.wpbeaverbuilder.com/article/422-add-a-singular-themer-layout-to-the-wordpress-page-template-field
<?php
/*
Template Name: Test Singular
Template Post Type: page, post
*/
//Substitute your own page ID in the following line
FLThemeBuilderLayoutRenderer::render_all( 88 );