Right now, we are only concerned with step one in this video, which ends at 1:11 (after 1:11, stop the video). When you create your account, skip the "Partner Group" section.
Why? Knotweeds are difficult to identify and photos are helpful. Later in the year, when you observe your knotweed plant(s), take some photos and upload them to iNaturalist (as seen in this video). Your photos are an important visual document that researchers can refer to later.
Because these plants are regulated, we are not able to plant or transport them. Instead, volunteers find opportunities to observe these plants where they are already growing. To help you locate a plant to observe, or to find out if this opportunity is a fit for you, review these web resources on wild parsnip and/or knotweed. (Most observers collect data on one or the other. A few observers are able to observe both kinds.)
After locating your plant in the real-world, add your site and add plants on Nature's Notebook. You can begin observing when you are ready.