Add the following to functions.php
For CSS
add_filter( 'fl_builder_render_css', function( $css, $nodes, $global_settings, $include_global ) {
$site_url = 'https://example.com';
$cdn_url = 'https://123456abcdef.cloudfront.net';
$css = str_replace( $site_url, $cdn_url, $css );
return $css;
}, 10, 4 );
For JS
add_filter( 'fl_builder_render_js', function( $js, $nodes, $global_settings, $include_global ) {
$site_url = 'https://example.com';
$cdn_url = 'https://123456abcdef.cloudfront.net';
$js = str_replace( $site_url, $cdn_url, $js );
return $js;
}, 10, 4 );