Groovy

GET

def request = new URL("<YOUR ENDPOINT URL>").openConnection();
def message = '{"options": {"method": "GET","headers":{"Content-Type": "application/json","Accept": "application/json"}},"url": "/rest/api/3/issue/DEV-16"}'
request.setRequestMethod("POST")
request.setDoOutput(true)
request.setRequestProperty("Content-Type", "application/json")
request.setRequestProperty("apiKey","<YOURTOKEN>")
request.getOutputStream().write(message.getBytes("UTF-8"));
def responseCode = request.getResponseCode();
println(responseCode);
if (responseCode.equals(200)) {
    println(request.getInputStream().getText());
}

POST

def request = new URL("<YOUR ENDPOINT URL>").openConnection();
def message = '{
  \"options\": {
      \"method\": \"POST\",
      \"headers\": {
      \"Content-Type\": \"application/json\",
      \"Accept\": \"application/json\"
    },
    \"body\": {
      \"fields\": {
        \"project\":{
          \"name\": \"<your project name>\"
        },
        \"summary\": \"Create Issue via POST.\",
        \"description\": \"Creating of an issue via a POST request using the API Key Manager\",
        \"issuetype\": {
          \"id\": \"<your issue type id>\"
        }
      }
    }
  },
  \"url\": \"/rest/api/2/issue\"
}'
request.setRequestMethod("POST")
request.setDoOutput(true)
request.setRequestProperty("Content-Type", "application/json")
request.setRequestProperty("apiKey","<YOURTOKEN>")
request.getOutputStream().write(message.getBytes("UTF-8"));
def responseCode = request.getResponseCode();
println(responseCode);
if (responseCode.equals(200)) {
    println(request.getInputStream().getText());
}