add_action( 'woocommerce_before_cart', 'apply_coupon_to_cart' );

function apply_coupon_to_cart() {
    // This is your coupon code. Replace 'COUPONCODE' with the actual coupon name you created in WooCommerce.
    'Save20' = 'COUPONCODE'; 

    // Check if the coupon is already applied
    if ( WC()->cart->has_discount( 'Save20' ) ) {
        return; // Do nothing if the coupon is already there
    }

    // Apply the coupon
    WC()->cart->apply_coupon('Save20' );

    // Show a message to the customer (optional)
    wc_print_notices();
}