It seems the api you are using expects all the parameters to be in the url:
Try:
local HTTPService = game:GetService('HttpService')
local text = game:GetService("HttpService"):EncodeUrl(text)
local Url = 'https://api.sightengine.com/1.0/text/check.json?text='..text.."&mode=standard&lang=en&opt%5Fcountries=gb&api%5Fuser=xxxx&api%5Fsecret=xxxxxxxxxxxxxxx" -- make sure to url encode your api_secret and api_user
local RequestData = {
['request'] = {
["text"] = "Hi, My name is Joe.",
["mode"] = 'standard',
['lang'] = 'en',
['opt_countries'] = 'gb',
['api_user'] = 'xxxxx',
['api_secret'] = 'xxxxxxxxxxxxxxx'
}
} -- we still use RequestData just in-case some properties are accessed from the post data.
local RequestData = HTTPService:JSONEncode(RequestData)
local Moderated = HTTPService:PostAsync(Url, RequestData)
print(Moderated)