在JSON中分配动态值

POST "WinHttp.WinHttpRequest.5.1" .如果我发送JSON,它就会工作,如1所示。但是,我想赋值并使用变量(2)进行调用。在不使用第三方libs的情况下,有没有办法做到这一点?

DYNAMIC="Some dynamic value will be here"

'#1 Following works
'body = "{ ""FIRST"": ""dynamic"", ""second"": ""somestring"", ""third"": 100}"

'#2 Folowing call fails.    
body = "{ ""FIRST"": DYNAMIC, ""second"": ""somestring"", ""third"": 100}"

TargetURL = "http://localhost:53518/api/Misc/myMethod/"
Set HTTPReq = CreateObject("WinHttp.WinHttpRequest.5.1")

HTTPReq.SetAutoLogonPolicy 0
HTTPReq.Open "POST", TargetURL, False
HTTPReq.setRequestHeader "Content-Type", "application/json"
HTTPReq.setRequestHeader "User-Agent", "Mozilla/5.0 (Windows NT 10.0; WOW64)AppleWebKit/537.36 (KHTML, like Gecko) Chrome/72.0.3626.121 Safari/537.36"
HTTPReq.setRequestHeader "Connection", "keep-alive"
HTTPReq.setRequestHeader "CacheControl", "no-cache"
HTTPReq.setRequestHeader "AcceptLanguage", "en-US,en;q=0.9,ht;q=0.8"
HTTPReq.setRequestHeader "AcceptEncoding", "gzip,deflate,br"
HTTPReq.setRequestHeader "Accept", "*/*"
HTTPReq.setRequestHeader "AcceptCharset", "UTF-8"
HTTPReq.send (body)