If your question is about how to generate iat and ext, you can do this using JMeter's built-in __groovy() function

• iat - ${__groovy((new Date().time / 1000).round(),)}
• ext - ${__groovy(((new Date().time + 7200) / 1000).round(),)}

More information on Groovy scripting in JMeter: Apache Groovy: What Is Groovy Used For?