register_authentication_field
The register_authentication_field can be used in combination with the authentication_form_input_name filter inside authentication_edit_form and authentication_new_form tags.
The register_authentication_field tag takes the name of a content field of the [authentication type] specified in the surrounding tag as an argument.
If the register_authentication_field tag is used, when the form it is used in will be submitted, it will also look for an input with the same name as used as input. It will store the data provided in that input as value for the content_field of the account that is created/updated.
Example:
Given that the authentication type "dealers" has a content field called "first_name", then the following piece of code will generate a form in which the visitor
{% authentication_edit_form "dealers" %}
{% authentication_field "email" %}
{% authentication_field "current_password" %}
{% authentication_field "password" %}
{% authentication_field "confirm_password" %}
<input type="text" name="{{ 'first_name' | authentication_form_input_name }}" />
{% register_authentication_field "first_name" %}
<input type="submit" value="Save">
{% endauthentication_edit_form %}