I wouldn't be surprised if the feature exists and I just haven't found it, but variables for everything from invoice amount to customer name could be used in templates. URL encoded versions can be used for API calls from ITFlow to the outside. Custom fields for outside API keys could also be added.
Example:
{customer_name} = Customer One
{url_customer_name} = Customer%20One
Use case: (This is my use case. There are many like it, but this one is mine.)
URL in ticket template to an outside form that lets customers pay by bank transfer. Form fields for invoice number, customer name, and amount will be supplied by variables.
"https://form.address/?invoice={invoice*number}&customer={url_customer_*name}&amount={url_invoice_amt}"
In this case the outside API key custom field is not necessary.