Hi,
Using below action hook you can show the field,
do_action( 'wcmp_afm_after_general_product_data', $post->ID, $product_object, $post );
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Or you can override templates/vendor-dashboard/product-manager/views/html-product-data-general.php template by copying it to yourtheme/dc-product-vendor/vendor-dashboard/product-manager/views/html-product-data-general.php
and do you custom code.
>> Regrading the other part, according to the present flow, we allow only the admin to add commission product variation wise from admin dashboard as you can see here (https://www.loom.com/share/83ae1e3932e34fa5ab6d6ecc3dfbae72).
Regards,