You can use Signup Prompt Customization along with the pre-user-registration trigger to add end-user supplied data (like a user’s phone number or location) from the signup prompt to user_metadata. Optionally, you can validate this data and show a validation error in the prompt.

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Prerequisites

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Add A Field To The Signup Prompt

Use the Management API to insert a custom field into a prompt using one of the Entry Points. This example adds the following content to the ulp-container-form-content-start insertion point: 
<div class="ulp-field">
  <label for="first-name">First Name</label>
  <input type="text" name="ulp-first-name" id="first-name">
</div>
The result is a First Name section in the signup prompt:

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Create An Action In The Pre-User Registration Trigger

You can build a custom pre-user registration action by going to Actions > Library > Build Custom.
Inside the code editor, update the onExecutePreUserRegistration handler: 
exports.onExecutePreUserRegistration = async (event, api) => {
  const firstName = event.request.body['ulp-first-name'];
  api.user.setUserMetadata("firstName", firstName);
};
Optionally, you can validate the user input and send a validation error by calling the api.validation.error method, then deploy the action: 
exports.onExecutePreUserRegistration = async (event, api) => {
  const firstName = event.request.body['ulp-first-name'];
  if(!firstName) {
    api.validation.error("invalid_payload", "Missing first name");
    return;
  }

  api.user.setUserMetadata("firstName", firstName);
};

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Add The Action To The Flow

Navigate to Actions > Flows > Pre User Registration > Add Action > Custom, then drag and drop your new action into the registration flow and select Apply.

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Test The Action

Sign up for an account in your test flow and leave the First Name field blank. You will see an error on submit:
After you enter a name in the First Name field, you are able to submit successfully:

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Verify That The Data Was Saved On user_metadata

Navigate to User Management > Users, then confirm the data has been saved by viewing the Details tab: