Rest API test with demo instance

So if I understand well, if I need to get a specific issue on ma tria servicedesk instance I need to run the following request :

 https://lieabertest.atlassian.net/rest/api/latest/issue/SD-3

My ServiceRequest SD-3 is existing on default trial instance

but when executing I get error below

"errorMessages": [  "Issue does not exist or you do not have permission to see it."],"errors": {}

Where do I pass the authentication user name and password to the request 
?

Thanks for clarification

I don't quite understand the question, but see the doc I pointed to for an explanation of the three parts of the url I said how to build.

So if I understand well, if I need to get a specific issue on ma trial servicedesk instance I need to run the following request :

https://lieabertest.atlassian.net/rest/servicedeskapi/request

 but when executing I get error below

<status><status-code>401</status-code><message>Client must be authenticated to access this resource.</message> </status>

Where do I pass the authentication user name and password to the request 
?

Thanks for clarification

That's probably authentication, as you suspect.  Start with a look at https://developer.atlassian.com/cloud/jira/platform/jira-rest-api-basic-authentication/  for that.  It covers the "basic" option, which I tend to stay away from apart from getting started with a bit of code, but there's links to the other options in the first paragraph.