Can I use a query string to append URLs in my campaigns?

Yes, you can modify a URL in your campaign to include a query string that can be used to automatically send mail merge field data to another website.

First you will need to obtain the exact mail merge field code to be used. While in an editable area, click Insert > Recipient Details > and select the desired mail merge field. For example, the correct mail merge code for the External ID field is [external_contact_id].

Then append the end of the URL as follows:
http://www.yourlinkhere.com/?NAME=[correctmailmergecode]

Or to continue the example from above using the External ID field (where userid is the name that you would designate based on how the site you are linking to would expect to receive such data):
http://www.yourlinkhere.com/?userid=[external_contact_id]

If the URL already has a query string, then you would append the URL with &userid=[external_contact_id].