Does it have to display? What if, instead, they were emailed the code directly? 

In any case, you'd need your list - with a column of "issued" or "unissued" - then just have an action to "query list" filtering to "unissued" -- then make sure you take the ID of that item and update it to "issued" right before emailing it / displaying it to your user.

Thanks - I think I can build such a workflow, however how would the workflow start? As far as I know, workflows only start once a list item is added or modified, so in this example what would trigger the query ton run, edit the field and email the token code to the user?

Sent from my iPhone

You could make it a Site Workflow, and then use a Start Form which would automatically pull in the user's name (who they're signed in as) and they'd just click a button which you could even label "Email Me My Token Now!" or something. 

Here's a post about start forms: