You can use wild cards such as {id} in the path for the endpoint designer e.g. https://clienturl.com/customer/{customer_id}/null.
We have an example of using wildcards and regular expressions in our Tyk Demo repository. You can also check out the Postman collection. The examples contain descriptions of what is happening.
You can also check out this thread about API key in path if you intend on manipulating the header or url in some way
Hope this helps.