The referral code displayed in the Shopify Admin's "Conversion summary" section is part of the landing_site data. The raw data may look something like this:

/?ref=blogpost

In the above URL example, the query parameter is called "ref" and the referral code itself is "blogpost".

EZ Exporter can extract just the value of the "ref" parameter (i.e. "blogpost") and place it in a separate column in the CSV file using this formula in the Calculated Fields section of the Data Settings:

get_url_query_param_value({{ landing_site }}, "ref")