Hey @tgriffiths thanks for your post! I can see that you have a support ticket open with Anna from our technical team, I will go ahead and share her response here as well just in case anyone else is looking for help with a similar request!

If you're looking to send lead data to a third-party server after a form is submitted, you can use webhooks: https://documentation.unbounce.com/hc/en-us/articles/203510044-Using-a-Webhook.
 
We also have the "Post form data to URL" option https://documentation.unbounce.com/hc/en-us/articles/203708344-Getting-Started-with-Integrations#content3. Out of these two, I recommend webhooks because the request and response for each lead sent via a webhook are recorded and available to view in the leads tables. This makes it easier to troubleshoot compared to "Post form data to a URL," where there are no logs.
 
It's not possible to add server-side scripts to Unbounce. We allow only frontend Javascript and HTML to be added to pages:
 
https://documentation.unbounce.com/hc/en-us/articles/203879070-Adding-Custom-JavaScript-and-CSS-in-the-Classic-Builder 
https://documentation.unbounce.com/hc/en-us/articles/360035250491-Adding-Your-Custom-Scripts-Using-Script-Manager 
https://documentation.unbounce.com/hc/en-us/articles/360034877112-Embedding-Custom-HTML-or-Widgets-into-Classic-Builder 
 
It is possible to send HTTP requests from Unbounce pages with scripts you add though.