Automatically add a coupon code on the checkout page

auto_add_coupon.php

/**
 * Apply Coupon Code automatically to the cart 
 */
function divi_engine_apply_discount_to_cart() {
    
    $order_total = WC()->cart->get_subtotal();
  
    if( $order_total > 50 ) { //Here you set the condition which in this case is spend over $50
        $coupon_code = '20%OFF'; //Here you indicate which coupon code to apply
        if ( !WC()->cart->add_discount( sanitize_text_field( $coupon_code ) ) ) {
            WC()->show_messages();
        }
    }
}
add_action( 'woocommerce_before_cart_table', 'divi_engine_apply_discount_to_cart' );