I had some challenging time getting basic authentication working with the Coinqvest RestAPI, but the following works for anyone wanting to do the some.
$basic = $key + ":" + $secret
$sha256 = [System.Security.Cryptography.HashAlgorithm]::Create('sha256');
$signature = $sha256.ComputeHash([System.Text.Encoding]::ASCII.GetBytes($basic))
$xbasic = [BitConverter]::ToString($signature).Replace('-','').ToLower()
$Header = @{'X-Basic' = $xbasic}
$URI = 'https://www.coinqvest.com/api/v1/auth-test'
Invoke-WebRequest -URI $URI -Header $Header