Hi Jeremy,
There is no API for URL builder because it's probably not really needed.
You should be able to build a string dynamically using these parameters listed on the URL Builder page.
utm_source
utm_medium
utm_term
utm_content
utm_campaign
All the builder does is fill in the bank variable values with what you are entering in the text fields above.
Example: [Website URL]?utm_source=[Campaign Source]&utm_medium=[Campaign Medium]&utm_campaign=[Campaign Name]
I hope it points you in the right direction.
Henning