If you are looking to add a custom attribute for the customer in your Magento 2 website and your Magento version is Adobe Commerce (Enterprise Version) then you can do it without writing a signal code. Adding a custom attribute for customers is natively provided by Adobe Commerce.
NOTE: Add custom attribute for the customer is supported by the Adobe Commerce version only. For Magento open source, you have to add this with custom code.
Below are steps to add a new custom attribute in Magento 2
Step 1: Go to the Magento 2 Admin > STORES > Attributes > Customer
Step 2: Now click on the “Add New Attribute” button.
Step 3: First fill up all the required details of the “Attribute Properties” section under the “Properties” tab.
From this section, you can choose the input type of attribute, label, default value etc. Also, you can choose where that attribute can be used means in customer segment, in search, in condition etc.
Step 4: Then fill out all the details of “Storefront Properties” under the same “Properties” tab.
In this section, you can choose whether you want to display this customer custom attribute to the store front-end and in which form you want to add this custom attribute.
Step 5: The final step is to set the label of the customer custom attribute for each store. For that navigate to the “Manage Label / Options” tab.
This is how you can add the customer attribute without any customization in Magento commerce.