I don't personally know about powershell, but httpie should be fine with spaces without needing the := syntax
$ http POST http://httpbin.org/post example-api-identifier="String with spaces"
yields
HTTP/1.1 200 OK
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Connection: close
Content-Length: 413
Content-Type: application/json
Date: Sat, 01 Feb 2020 00:25:41 GMT
Server: gunicorn/19.9.0
{
"args": {},
"data": "{\"example-api-identifier\": \"String with spaces\"}",
"files": {},
"form": {},
"headers": {
"Accept": "application/json, */*",
"Accept-Encoding": "gzip, deflate",
"Connection": "keep-alive",
"Content-Length": "48",
"Content-Type": "application/json",
"Host": "httpbin.org",
"User-Agent": "HTTPie/1.0.0"
},
"json": {
"example-api-identifier": "String with spaces"
},
"origin": "127.0.0.1",
"url": "http://httpbin.org/post"
}