Magento – Magento 2 – separate registration form to register into customer group with custom field

To use that field at customer registration, you need to enable all the other address fields and then add a addtional field for "vat_id" to /vendor/magento/module-customer/view/frontend/templates/form/register.phtml.

Then you need to add validation if you want the field to be required.

In the phtml file there is <?php if ($block->getShowAddressFields()): ?> change this to <?php if (true): ?> to show the address fields.

Then add a new field:

<div class="field vat_id required">
            <label for="vat_id" class="label"><span><?= $block->escapeHtml(__('VAT Number')) ?></span></label>
            <div class="control">
                <input type="text" name="vat_id" value="<?= $block->escapeHtmlAttr($block->getFormData()->getVatId()) ?>" title="<?= $block->escapeHtmlAttr(__('VAT Number')) ?>" id="vat_id" class="input-text required <?= $block->escapeHtmlAttr($this->helper('Magento\Customer\Helper\Address')->getAttributeValidationClass('vat_id')) ?>">
            </div>
        </div>

Of course you should use an own template file (in your theme or module) for this, instead of the core file.

Another possibility is https://www.geissweb.de/eu-vat-enhanced-for-magento-2.html which has a config option to show the address fields and the VAT number field at customer registration. You can also add validation to the field with the module, its options are:

• Optional
• Optional, but require a valid VAT number when filled
• Require a valid VAT number, validated through the validation service