Here is an example that works in Powershell - for those of us using it :P
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clear
$url = "https://plex.tv/users/sign_in.xml"
$BB = [System.Text.Encoding]::UTF8.GetBytes("myplexaccount:mypassword")
$EncodedPassword = [System.Convert]::ToBase64String($BB)
$headers = @{}
$headers.Add("Authorization","Basic $($EncodedPassword)") | out-null
$headers.Add("X-Plex-Client-Identifier","TESTSCRIPTV1") | Out-Null
$headers.Add("X-Plex-Product","Test script") | Out-Null
$headers.Add("X-Plex-Version","V1") | Out-Null
[xml]$res = Invoke-RestMethod -Headers:$headers -Method Post -Uri:$url
$token = $res.user.authenticationtoken
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Maybe their is a better place to put this, but, this seemed like a good one to store examples to help others along.
Cade