{"@type":"login",
 "username":"xxx@gmail.com",
 "password":"yyy"
}

$params = @{"@type"="login";
 "username"="xxx@gmail.com";
 "password"="yyy";
}

Invoke-WebRequest -Uri http://foobar.com/endpoint -Method POST -Body $params

$JSON = @'
{"@type":"login",
 "username":"xxx@gmail.com",
 "password":"yyy"
}
'@

$response = Invoke-RestMethod -Uri "http://somesite.com/oneendpoint" -Method Post -Body $JSON -ContentType "application/json"

$response = Invoke-WebRequest -Uri "http://somesite.com/oneendpoint" -Method Post -Body $JSON -ContentType "application/json"

$JSON = @{
    "@type" = "login"
    "username" = "xxx@gmail.com"
    "password" = "yyy"
} | ConvertTo-Json