WooCommerce registration custom fields

functions.php

<?php

// Add new fields in the registration form
// @link https://www.cloudways.com/blog/add-woocommerce-registration-form-fields/
add_action( 'woocommerce_register_form_start', function() { ?>
	<p class="form-row form-row-wide">
		<label for="reg_billing_phone"><?php _e( 'Phone', 'woocommerce' ); ?></label>
		<input type="text" class="input-text" name="billing_phone" id="reg_billing_phone" value="<?php esc_attr_e( $_POST['billing_phone'] ); ?>" />
	</p>

	<p class="form-row form-row-first">
		<label for="reg_billing_first_name"><?php _e( 'First name', 'woocommerce' ); ?><span class="required">*</span></label>
		<input type="text" class="input-text" name="billing_first_name" id="reg_billing_first_name" value="<?php if ( ! empty( $_POST['billing_first_name'] ) ) esc_attr_e( $_POST['billing_first_name'] ); ?>" />
	</p>

	<p class="form-row form-row-last">
		<label for="reg_billing_last_name"><?php _e( 'Last name', 'woocommerce' ); ?><span class="required">*</span></label>
		<input type="text" class="input-text" name="billing_last_name" id="reg_billing_last_name" value="<?php if ( ! empty( $_POST['billing_last_name'] ) ) esc_attr_e( $_POST['billing_last_name'] ); ?>" />
	</p>
	<div class="clear"></div>
<?php } );

// Validate new fields
add_action( 'woocommerce_register_post', function( $username, $email, $validation_errors ) {
	if ( isset( $_POST['billing_first_name'] ) && empty( $_POST['billing_first_name'] ) ) {
		$validation_errors->add( 'billing_first_name_error', __( '<strong>Error</strong>: First name is required!', 'woocommerce' ) );
	}

	if ( isset( $_POST['billing_last_name'] ) && empty( $_POST['billing_last_name'] ) ) {
		$validation_errors->add( 'billing_last_name_error', __( '<strong>Error</strong>: Last name is required!.', 'woocommerce' ) );
	}

	return $validation_errors;
}
, 10, 3 );

// Save fields
add_action( 'woocommerce_created_customer', function ( $customer_id ) {
	if ( isset( $_POST['billing_phone'] ) ) {
		// Phone input field which is used in WooCommerce
		update_user_meta( $customer_id, 'billing_phone', sanitize_text_field( $_POST['billing_phone'] ) );
	}

	if ( isset( $_POST['billing_first_name'] ) ) {
		//First name field which is by default
		update_user_meta( $customer_id, 'first_name', sanitize_text_field( $_POST['billing_first_name'] ) );
		// First name field which is used in WooCommerce
		update_user_meta( $customer_id, 'billing_first_name', sanitize_text_field( $_POST['billing_first_name'] ) );
	}

	if ( isset( $_POST['billing_last_name'] ) ) {
		// Last name field which is by default
		update_user_meta( $customer_id, 'last_name', sanitize_text_field( $_POST['billing_last_name'] ) );
		// Last name field which is used in WooCommerce
		update_user_meta( $customer_id, 'billing_last_name', sanitize_text_field( $_POST['billing_last_name'] ) );
	}
} );

Custom Fields Registration For Woocommerce that also provide easy setup with option