I need to add my ACF field to a custom function. I have tried this but there is syntax error.
add_action( 'flatsome_custom_single_product_1', function () {
echo <?php the_field('designer'); ?>;
} );
When you are calling the_field('designer') outside the actual post loop, you need to pass the post ID as well. Here is the Ref.
For your case, you can achieve this by using the code like below
add_action( 'flatsome_custom_single_product_1', function () {
global $post;
echo get_field('designer', $post->ID);
} );