If you provide an example of the response from that endpoint, can you identify what you want, but you don't know how to create the Json path for it?
In that case, please provide an example of the output and what you are trying to extract from it. https://www.javainuse.com/jsonpath is the tool I normally use to work through the jsonpath I need in my translation map.
If the issue is that the endpoint return a lot of details and you don't know which route is the right one, then this room may not be able to help you. There is a general platform room that might be able to help you, or you might need to contact support in order to figure out how to accomplish your goal.
I have never used this API, so I am just guessing based on the documentation... Can you pass othertestemail@testdomain.com in as the "pattern" query parameter to narrow down the results to only the correct entry?
Assuming that works to filter the results, then something like
entities[0].fromName
in your translationMap should get what you are looking for.