Here are 2 easy steps to create customer attributes in Magento 2:

Step 1. Install Magento 2 customer attributes extension by MageDelight

Step 2. Click the Add New Attribute button at the top of the admin panel to insert a new customer attribute

Read this How to Configure Customer Attribute in Magento 2 guide to setup this extension.

Here are the few more resource to help you get most out of this Magento 2 Custom Attributes Extension.

• How to Set Custom Validation Message in Magento 2
• How to Create Customer Attribute in Magento 2 Programmatically