ConvertFrom-JWT.ps1

function Invoke-SplitJWT {
        Param
        (
            [Parameter(Mandatory = $true,
                ValueFromPipeline = $true,
                Position = 0)]
            $String
        )
    
        Process {
            $Length = $String.Length
            if ($String.Length % 4 -ne 0) {
                $Length += 4 - ($String.Length % 4)
            }
            return $String.PadRight($Length, "=")
        }
    }

    function ConvertFrom-JWT {
        [CmdletBinding()]
        Param
        (
            [Parameter(Mandatory = $true,
                ValueFromPipeline = $true,
                Position = 0)]
            $Base64JWT
        )
    
        Begin {
        }
        Process {
            $Spl = $Base64JWT.Split(".")
            [PSCustomObject] @{
                Header  = [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String((Invoke-SplitJWT $Spl[0]))) | ConvertFrom-Json
                Payload = [System.Text.Encoding]::UTF8.GetString([System.Convert]::FromBase64String((Invoke-SplitJWT $Spl[1]))) | ConvertFrom-Json
            }
        }
        End {
        }
    }

Example

( ConvertFrom-JWT -Base64JWT "<the graph token>" ).payload