The intermediate(s) needed are included in the API response from Let's Encrypt, along with the certificate. It shouldn't be hardcoded into the script.
I assume you're using this program: GitHub - BlueFeatherGroup/FileMaker-LetsEncrypt-Win: A PowerShell script for fetching and renewing Let's Encrypt SSL certificates for FileMaker Server running on Windows Server.
It looks like this repository has been abandoned.
I haven't spent much time looking, but it appears somebody else has taken up maintenance of getSSL.ps1 here. Perhaps it will work better for you: GitHub - dansmith65/FileMaker-LetsEncrypt-Win: A PowerShell script for fetching and renewing Let's Encrypt SSL certificates for FileMaker Server running on Windows Server.
If you do manually need to get intermediates, they are always posted here: Chain of Trust - Let's Encrypt
But you shouldn't use them manually, as we have multiple online intermediates so you can't know ahead of time which we'll use.