KeyframeSequence

local function getSequenceLength(keyframeSequence)
	local length = 0
	for _, keyframe in pairs(keyframeSequence:GetKeyframes()) do
		if keyframe.Time > length then
			length = keyframe.Time
		end
	end
	return length
end

local keyframeSequence = Instance.new("KeyframeSequence")

getSequenceLength(keyframeSequence)

local KeyframeSequenceProvider = game:GetService("KeyframeSequenceProvider")

local function createPreviewAnimation(keyframeSequence)
	local hashId = KeyframeSequenceProvider:RegisterKeyframeSequence(keyframeSequence)
	local Animation = Instance.new("Animation")
	Animation.AnimationId = hashId
	return Animation
end

local keyframeSequence = Instance.new("KeyframeSequence")

local animation = createPreviewAnimation(keyframeSequence)

print(animation)

-- create the keyframesequence
local keyframeSequence = Instance.new("KeyframeSequence")
keyframeSequence.Loop = false
keyframeSequence.Priority = Enum.AnimationPriority.Action

--  create a keyframe
local keyframe = Instance.new("Keyframe")
keyframe.Time = 0

-- create sample poses
local rootPose = Instance.new("Pose")
rootPose.Name = "HumanoidRootPart"
rootPose.Weight = 0

local lowerTorsoPose = Instance.new("Pose")
lowerTorsoPose.Name = "LowerTorso"
lowerTorsoPose.Weight = 1

-- set the sequence hierarchy
rootPose:AddSubPose(lowerTorsoPose) -- lowerTorsoPose.Parent = rootPose
keyframe:AddPose(rootPose) -- rootPose.Parent = keyframe
keyframeSequence:AddKeyframe(keyframe) -- keyframe.Parent = keyframeSequence

-- parent the sequence
keyframeSequence.Parent = workspace