Hi,

When adding a custom field to a Service Item you have multiple options for field types. The second to last one is URL:

If you need any additional assistance please let me know!

That field is for requester to add a URL in to the form.
If you like to add a url for the requester to click on you need to use the Content field.
The last option on the bar. 

It seems the “URL” custom field in the Service Catalog is treated as plain text by default. You might need to use a custom HTML field or description section to add clickable links. Alternatively, check if your plan or settings support rich text or hyperlink formatting for custom fields.