"Unable to cast string to token" error while attempting PostAsync?

local http = game:GetService('HttpService')
local urlEncode = function(str) return http:UrlEncode(str) end
local POST = function(url,data) return http:PostAsync(url,data,'ApplicationUrlEncoded') end
local Url = 'https://discordapp.com/api/webhooks/407404186616201216/dcTEEdeNuoIsm3Uwz0ygU7tGx96k2faHYUtBwTdAVyl-4m6YefDNSccvzE96pyz-kH3y'

http:PostAsync(
    Url,
    'contents='..('a'),
    'HttpUrlEncoded'
)

I'm attempting to send a post request every time someone joins my game. The issue that I'm having is that I'm pretty sure this code should work, but when I run it I get the error mentioned in the title. If anyone can help me with this I would really like that. I'm not sure if the data for the post should be a table or a string. From what I read it should be a string.