I installed a plugin on my website that handles single sign-on to Access Control Services (ACS). As part of the setup, there is a space to enter “X509 certificate thumbprint (used for issuer URL token signing) – The token signing certificate thumbprint”. What am I supposed to enter in there?

The certificate thumbprint is a hash of the public key of the certificate (Wiki: Public key certificate). Comodo has a HowTo for retrieving it: Comodo.
With openssl: