When to attempt your first test

In a few weeks I’ll be finishing an HM plan and will start my first of your triathlon plans. It’s been some time since I’ve either swum or seriously biked. Knowing that establishing your FTP is critical to your training, when should I perform my first one. Having been through a running plan a couple times I’m at least familiar with perceived efforts at certain zones. Should I perform the first few weeks using PE as a guide to establish some initial fitness before attempting a test or should I jump right in and establish a cycling and swimming threshold immediately?