functions.php

add_action( 'woocommerce_product_options_general_product_data', 'wc_custom_add_custom_fields' );
function wc_custom_add_custom_fields() {
// Print a custom text field
woocommerce_wp_text_input( array(
'id' => '_custom_text_field1',
'label' => 'Name',
'description' => ' Field, you can write your Name.',
'desc_tip' => 'true',
'placeholder' => 'Ram'
) );
}