public with sharing class GetIssues {

 public static void Getfields(String JIRA_Key) {

    HttpRequest req = new HttpRequest();
    HttpResponse res = new HttpResponse();
    Http http = new Http();

    String username = 'admin';
    String password = 'admin';


    Blob headerValue = Blob.valueOf(username+':'+password);
    String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
    req.setHeader('Authorization', authorizationHeader);
    req.setHeader('Content-Type','application/json');

    String endpoint = 'https://mysfjira.atlassian.net/rest/api/2/issue/'+JIRA_Key;

    req.setMethod('GET');
    req.setEndpoint(endpoint);
    res = http.send(req);

   }
}