Property Transfer Between Test Cases

If I am getting the scenario correctly, here is a solution I can offer.

make your endpoint url as a variable taken from project/suite property as in
${#Project#PIEndpointURL_REST}

Then, when u capture that id value in variable, define another string variable, and append this id. eg:
def url = context.expand(' ${#Project#PIEndpointURL_REST}')
def temp = url + id
log.info temp

Now as the last step, using groovy script, write this new value back to the project/suite property which is used as endpoint url. Like if u r defining the URl as a testsuite prop, then:

def ts = testRunner.testCase.testSuite.project.testSuites[<yourTestSuiteName>]
tc.setPropertyValue("PIEndpointURL_REST", temp)

Now next json request will take this newly written url with id as its endpoint.
Let me know if it helps.
All the best

Regards,
Ankita