Thanks in advance.
Does your third party application offer an api to get the service ticket? If so, it might be even easier to call this webservice via the REST/SOPA microflow action.
Let me know if you have any questions.
Hope this helps!
In that case, you can simply use the call REST service action in your microflow.
Create a entity with an attribute which can hold the key.
Open a page with the editable entity and add the html snippet below.