When creating a .pfx, you'll need a copy of the private key from your server, as well as the .crt file that you downloaded from your SSL provider. You can use OpenSSL commands in the command line to create the PFX. If you want to understand what OpenSSL is, you can find more info here - https://www.openssl.org/ and You can install OpenSSL from here: openssl
I'm including a sample below:
openssl pkcs12 -export -out certificate.pfx -inkey private.key -in certificate.crt -passout pass:XXXXX
This will create a certificate.pfx file from your private key, as well as the .crt you downloaded. You'll just need to make sure that you update the names in the sample code above to match your certificate/private key information. Also replace the password "XXXXX" with the password you need