In the Certificate File field, browse to the third-party certificate that includes a subject alternative name (SAN) that ...

In the Certificate File field, browse to the third-party certificate that includes a subject alternative name (SAN) that matches the external FQDN of the AD FS server. This certificate needs to include a private key. In the Subject Name field, select the SAN you want to use, for example sts.contoso.com. Select Next.