Custom data not saving to members on checkout

Question

I'm trying to save some custom member fields to EE's member section using CartThrob's checkout form.

I have created all the necessary member fields and mapped some of the basic fields in CartThrob's settings and they save just fine. Only the custom_data inputs don't save in the newly created EE member.

Here's the gist of what my form looks like so far:

{exp:cartthrob:checkout_form create_user="yes" group_id="5"}    
    <input type="text" name="first_name" value="" id="first_name" class="required"  />
    <input type="text" name="last_name" value="" id="last_name" class="required"  />
    <input type="text" value="" placeholder="Preferred Name" name="custom_data[preferred_name]" />
    <input type="text" value="" placeholder="DOB Day" name="custom_data[member_dob_day]" />
{/exp:cartthrob:checkout_form}

This was all done following CartThrob's docs and many iterations of not working. Any direction guys, much appreciated.

Answer 1

You might find this post useful which shows how we approached this scenario.

Cartthrob checkout form with different billing and member information

Answer 2

I believe the id is missing in your custom_data inputs.

Try it like this:

<input name="custom_data[whatever]" id="custom_data[whatever]" />