Hi,
I've not used it yet but the endpoint below seems to be what you need:
"/issuelink/{issueKey}/testcases"
DESCRIPTION
Retrieve all Test Cases linked to an Issue.
PARAMETERS
issueKeyrequired, string (The key of the linked Issue)
The above endpoint seems come from Zephyr Scale server version, not the Zephyr Scale cloud version. I have tried but I got 'Resource identified by path '/issuelink/JIRATEST2-188/testcases', does not exist. The JIRATEST2-188 is one of my jira issue key, the full URL path I used is "https://api.zephyrscale.smartbear.com/issuelink/JIRATEST2-188/testcases"
I don't know if there's a difference in using the API between Cloud and Server (I'm on the Server version) but I have to prefix the endpoints with this URL...
https://jira.[MySite].com/rest/atm/1.0/
An example of how it works for the testresults endpoint which I have used before successfully
https://jira.[MySite].com/rest/atm/1.0/testrun/[MyKey]/testresults
And an example of how I think the endpoint below would work (but I've not tested)
https://jira.[MySite].com/rest/atm/1.0/issuelink/[issueKey]/testcases
Good luck.
