internal/functions/resolve/Resolve-ServicePrincipal.ps1

function Resolve-ServicePrincipal
{
    [CmdletBinding()]
    Param (
        [Parameter(Mandatory = $true)]
        [string] $InputReference,
        [switch] $DontFailIfNotExisting,
        [switch] $SearchInDesiredConfiguration,
        [System.Management.Automation.PSCmdlet]
        $Cmdlet = $PSCmdlet
    )
    
    begin {
        $InputReference = Resolve-String -Text $InputReference
    }
    process
    {
        try {
            if ($InputReference -match $script:guidRegex) {
                $servicePrincipal = (Invoke-MgGraphRequest -Method GET -Uri ("$script:graphBaseUrl/servicePrincipals/{0}" -f $InputReference)).Id
            }
            else {
                $servicePrincipal = (Invoke-MgGraphRequest -Method GET -Uri ("$script:graphBaseUrl/servicePrincipals/?`$filter=(displayName eq '{0}') and (servicePrincipalType eq 'Application')" -f $InputReference)).Value.Id
            }

            if (-Not $servicePrincipal -and $SearchInDesiredConfiguration) {
                if ($InputReference -in $script:desiredConfiguration["servicePrincipals"].displayName) {
                    $servicePrincipal = $InputReference
                }
            }

            if (-Not $servicePrincipal -and -Not $DontFailIfNotExisting) { throw "Cannot find servicePrincipal $InputReference" } 
            elseif (-Not $servicePrincipal -and $DontFailIfNotExisting) { return }

            if ($servicePrincipal.count -gt 1) { throw "Got multiple servicePrincipals for $InputReference" }
            return $servicePrincipal
        }
        catch {
            Write-PSFMessage -Level Warning -String 'TMF.CannotResolveResource' -StringValues "servicePrincipal" -Tag 'failed' -ErrorRecord $_
            $Cmdlet.ThrowTerminatingError($_)                
        }            
    }
}