How to add contact seller button in product page

You can use woocommerce_single_product_summary hook to add custom button in the single product page in WooCommerce. Please check following example. Custom link added and button class is given to display it like a button.

add_action( 'woocommerce_single_product_summary', 'wpso_add_seller_button', 15 );
function wpso_add_seller_button() {
    echo '<a href="https://example.com" class="button">Contact Seller</a>';
}