Membership plugin - custom field for sign up form

Hi,
I've installed this plugin to my site. I plan to set it up so there are different subscriptions available that you can pay monthly for that will give you access to certain content on the site. As such I am trying to add a membership type field to the sign up form. I am using the below code but it's not working

I am successfully pulling the membership list from the CMS for the dropdown. The form submits fine and sets up the user, but it doesn't save the membership type against the user

<select name="membership_status" id="select">
<?php foreach ($membership_plansRecords as $record): ?>
<option value="<?php echo htmlencode(@$_REQUEST['membership_status']); ?>"><?php echo htmlencode($record['title']) ?></option>
<?php endforeach ?></select>