Another way of doing this is by loading your certificate directly from your certificate store using PS Providers. Use Get-PSProviders to determine available PSProviders on your machine.
Once you have cert provider loaded, you can now get the certificate using Get-ChildItem
Launch certmgr.msc from run to launch the certificate store
Assuming that your certificate is stored under Personal folder in your cert store
and has "Company Name" set in the subject property of the certificate, and there is only certificate in that folder with Company Name in the subject - you can get the certificate like so
$my_cert = Get-ChildItem cert:\CurrentUser\My | ? {$_.Subject -match "Company Name"}
$my_cert will be your certificate object that you can pass directly to Set-AuthenticodeSignature cmdlet
Set-AuthenticodeSignature -Certificate $my_cert -FilePath fqn_to_dll.dll -Timestampserver "http://timestampurl"
post signing, you can retrieve the sign status by querying on the Status property for "Valid" or not like
$result = Set-AuthenticodeSignature -Certificate $my_cert -FilePath fqn_to_dll.dll -Timestampserver "http://timestampurl" | Select Status
if(-Not ($result -eq "Valid")){
Write-Output "Error Signing file: Status: $($result.Status)"
}