@Joe Williams you shouldn't need the blank $data variable. the $requestVars should just have the $data removed when you're doing a GET. There's two different script samples, one is for a GET and one is for a POST.
The issue it looks like what you have above is your $queryParams, this needs to be the way it is in your second example. The auth fails because you're including them in the resourcePath information and that's breaking the authentication hash. In the code you posted second, it should look like this. Hope this helps!
<# request details #>
$httpVerb = 'GET'
$resourcePath = '/setting/collectors'
$queryParams = '?fields=id,description,hostname'
<# Construct URL #>
$url = 'https://' + $company + '.logicmonitor.com/santaba/rest' + $resourcePath + $queryVars
<# Get current time in milliseconds #>
$epoch = [Math]::Round((New-TimeSpan -start (Get-Date -Date "1/1/1970") -end (Get-Date).ToUniversalTime()).TotalMilliseconds)
<# Concatenate Request Details #>
$requestVars = $httpVerb + $epoch + $resourcePath
<# Construct Signature #>
$hmac = New-Object System.Security.Cryptography.HMACSHA256
$hmac.Key = [Text.Encoding]::UTF8.GetBytes($accessKey)
$signatureBytes = $hmac.ComputeHash([Text.Encoding]::UTF8.GetBytes($requestVars))
$signatureHex = [System.BitConverter]::ToString($signatureBytes) -replace '-'
$signature = [System.Convert]::ToBase64String([System.Text.Encoding]::UTF8.GetBytes($signatureHex.ToLower()))
<# Construct Headers #>
$auth = 'LMv1 ' + $accessId + ':' + $signature + ':' + $epoch
$headers = New-Object "System.Collections.Generic.Dictionary[[String],[String]]"
$headers.Add("Authorization", $auth)
$headers.Add("Content-Type", 'application/json')
<# Make Request #>
$response = Invoke-RestMethod -Uri $url -Method $httpVerb -Header $headers
<# Print status and body of response #>
$status = $response.status
$body = $response.data
Expert
