Paste a PKCS#10 certificate signing request and see exactly what you're about to send your CA: subject, key type, requested names and extensions, attributes — plus a real check of the self-signature, done with your browser's own crypto. CSRs carry organisation details and sometimes plaintext challenge passwords; here, nothing you paste leaves this page.
BEGIN CERTIFICATE REQUEST (and NEW CERTIFICATE REQUEST) blocks, or bare base64 DER. Certificates and private keys are recognised and refused — certificates belong in the certificate decoder.