how to achieve this look for my horse's tail?

A lot of horses who have tails like that did not look like that at the barn. There is probably false tail fastened in there.

I do not agree with the people who have answered above me.

The horse pictured does not have a fake tail on. Warmbloods and drafts commonly have long, thick tails. In addition, fake tails are never used in vaulting.

Braiding, brushing, and constant shampooing with remove MANY hairs that you want to keep in. Instead, shampoo and condition once a month, at most. After washing, remove any tangles by hand and use a leave in condition. Do not use spray, as it won't last. Get something from a squeeze bottle. I suggest not braiding your horse's tail. It can easily get caught in something, even in the stall, and will remove hairs.

Like the horse in the picture, you can "bang" your horse's tail to give the illusion of a thicker tail. Run your hand down the tail and stop at the end, clipping off about a centimeter of hair in a straight line.

Unless you are willing to brush their tail several times a day, put spray in conditioner in once a day and then twice a week shampoo and condition it there is no way to keep it like that. It takes a lot of work just to keep it like that for a show. Horses are not feminine animals and I honestly don't think they mind getting crap in it. For show horses, their tail is kept braided full time except for once a week when it is shampooed brushed and dried. Oh an by the way, there are no extensions involved here, only TLC. It's really not that hard. Just stay on top of it, keep the horse in their stall unless it's braided.