Use Invoke-WebRequest with a username and password for basic authentication on the GitHub API

I am assuming Basic authentication here.

$cred = Get-Credential
Invoke-WebRequest -Uri 'https://whatever' -Credential $cred

You can get your credential through other means (Import-Clixml, etc.), but it does have to be a [PSCredential] object.

Edit based on comments:

GitHub is breaking RFC as they explain in the link you provided:

The API supports Basic Authentication as defined in RFC2617 with a few slight differences. The main difference is that the RFC requires unauthenticated requests to be answered with 401 Unauthorized responses. In many places, this would disclose the existence of user data. Instead, the GitHub API responds with 404 Not Found. This may cause problems for HTTP libraries that assume a 401 Unauthorized response. The solution is to manually craft the Authorization header.

Powershell's Invoke-WebRequest does to my knowledge wait for a 401 response before sending the credentials, and since GitHub never provides one, your credentials will never be sent.

Manually build the headers

Instead you'll have to create the basic auth headers yourself.

Basic authentication takes a string that consists of the username and password separated by a colon user:pass and then sends the Base64 encoded result of that.

Code like this should work:

$user = 'user'
$pass = 'pass'

$pair = "$($user):$($pass)"

$encodedCreds = [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes($pair))

$basicAuthValue = "Basic $encodedCreds"

$Headers = @{
    Authorization = $basicAuthValue
}

Invoke-WebRequest -Uri 'https://whatever' -Headers $Headers

You could combine some of the string concatenation but I wanted to break it out to make it clearer.

Use this:

$root = 'REST_SERVICE_URL'
$user = "user"
$pass= "password"
$secpasswd = ConvertTo-SecureString $pass -AsPlainText -Force
$credential = New-Object System.Management.Automation.PSCredential($user, $secpasswd)

$result = Invoke-RestMethod $root -Credential $credential

I had to do this to get it to work:

$pair = "$($user):$($pass)"
$encodedCredentials = [System.Convert]::ToBase64String([System.Text.Encoding]::ASCII.GetBytes($Pair))
$headers = @{ Authorization = "Basic $encodedCredentials" }
Invoke-WebRequest -Uri $url -Method Get -Headers $headers -OutFile Config.html