Following is a sample header for authenticating with Azure. Generate an authentication header, sometimes called a Bearer token, and provide the REST API URI to connect to with any parameters or a Request Body

get-azure-bearertoken.ps1

# Login first with Connect-AzureRmAccount if not using Cloud Shell

$azContext = Get-AzureRmContext
$azProfile = [Microsoft.Azure.Commands.Common.Authentication.Abstractions.AzureRmProfileProvider]::Instance.Profile
$profileClient = New-Object -TypeName Microsoft.Azure.Commands.ResourceManager.Common.RMProfileClient -ArgumentList ($azProfile)
$token = $profileClient.AcquireAccessToken($azContext.Subscription.TenantId)
$authHeader = @{
    'Content-Type'='application/json'
    'Authorization'='Bearer ' + $token.AccessToken
}

$authHeader

# Invoke the REST API
# $restUri = 'https://management.azure.com/subscriptions/{subscriptionId}?api-version=2016-06-01'
# $response = Invoke-RestMethod -Uri $restUri -Method Get -Headers $authHeader