Secret identifier can be found on Azure Portal.
Secret value must be a Base64 encoded PKCS#12 file with content type set to application/x-pkcs12
.
It must also be protected by a non-empty password(both for private key and PFX container) due to restriction of Java security library.
When uploading to Azure, choose Manual as upload option to avoid password being stripped.
You can convert your certificate to PKCS#12 format using OpenSSL:
openssl pkcs12 -inkey key.pem -in cert.pem -export -out cert.pfx
base64 < cert.pfx > cert.pfx.b64
Then you can paste the content of cert.pfx.b64
into secret value field on Azure Portal.