Assign a value based on URL variable

Is it possible to assign a value to a CB field based on a URL variable during registration?

I have 2 types of members on a site I am creating and I would like to automatically assign a value to a field during registration based on a variable I can include in the URL of the registration page like:

There are just 2 types of membership on this site, but I don't want guests selecting their type of membership on the form.

If there is a better method to achieve this, please suggest it.

Thanks.

If it's displayed on the registration form, yes. You can set the inputs value using jQuery in a Code action on the registration form display trigger. Otherwise no, because that field won't exist in the HTML form POST data.

Can the field be set to hidden to ensure registrants do not change the field value and still have this work? I tried setting the field to read only, but it could still be modified on registration.
Thanks for your help.

You can probably add the CSS Class "hidden" (without quotes) to the field in CB > Field Management to hide its row as that's a core Bootstrap 3 class. Read Only fields can still be edited during registration. Note you can not turn off registration edit for what you're wanting to work.

Your only other alternative is to use the before registration form display trigger and in a Code action with method set to jQuery grab the value from the URL and dynamically add a hidden input to the registration form. Next you need an after registration trigger that'll store that hidden input to a CB field.

Either usage should work fine, but they both are going to take some configuration. First usage should only need 1 action to populate the field. Second usage takes 2. Neither are particularly super secure as anyone could just inspect the HTML and alter the form values. Second usage is a little better than first in that regard.