I like growing my leeks from seed, and I tend to start them about the same time that I start onion and shallot seeds, right about the beginning of February for me here in the Oregon Willamette Valley. You will need to check your proper time of starting the seeds in your area.
Much like I do with onions, I start a dense planting of seeds, so you get a tight cluster of seedlings coming up all at once, and when they become large enough for transplanting you very gently separate the seedlings from one another. Take care to not harm the roots as you disentangle the roots from each other. You can trim the roots a bit if they are very long, otherwise just make sure they are all covered up and not exposed. If they are to long to achieve that easily, then snip off the ends.
When you grow leeks in ground, you plant the seedlings in a furrow about 6″ deep, add just enough soil to cover the roots. Then as the seedlings grow up, you add soil until it’s even with the soil line.
When you grow leeks in an Earthbox you can replicate this as much as possible by simply just filling the box half way up when you first prep it. Like in the photo below.
As the tiny little seedlings grow up, you add a little bit of soil, bit by bit. In the photo below you can see the leeks 2 months after they were transplanted, with the soil filled up much closer to the surface. After taking this photo, I filled the box all the way up to the top of the box.
And four months after planting, having been fully filled to the top for a while and now ready for harvest.
As you can see here, growing them this way in an Earthbox produces a nicely blanched leek, just like you get with furrow grown in-ground planted leeks.