In an X.500 distinguished name the "keys" represent Object Identifiers abbreviated OIDs. This includes the Subject and Issuer names in an X.509 certificate and also the Subject name in a PKCS10 CSR, and quite a few other things.
I could not find a complete list of the allowed keys.
There is no complete list. The OID assignment scheme allows unlimited hierarchical expansion -- this is like asking for a complete list of all URLs in the world. OpenSSL predefines a fixed but large set of OIDs that have been standardized or at least widely used; see the header obj_mac.h on your system if you have a 'development' version of OpenSSL installed (i.e. headers and libraries suitable for compiling your own code, rather than merely running precompiled code) or if you have source code the crypto/objects.txt file from which obj_mac.h is derived or in the repository. In the former file the LN_ string is the 'long name' and SN_ is the 'short name' or abbreviation, if different. In the latter file these are just columns on one line, with the short name first or empty to indicate there is no distinct abbreviation.
For example CN is the short name and commonName the long name for the OID 'X509 3' which, using other definitions in the file, expands to the actual OID 2.5.4.3.
If you -- or (more likely) someone or something you are interoperating with -- needs an OID outside of OpenSSL's predefined set, either because it's standard but rare and/or new, or because it's not standard at all, then you can still use it in OpenSSL by defining it in your configuration file; see the oid_file and oid_section items in the man page for req or for ca or the generic config for other commands like x509.
Typically the OIDs defined in X.509, and those defined for X.509 in PKCS9 (such as emailAddress), are commonly used in X.509 subjects, but technically any OID can be.
As you can see in the predefined list(s), there is an X509-scope OID for long name organizationIdentifier with no different short name -- this MAY be what your specifier wants, although it may not. There is no such obviously applicable OID for a URI, or URL; ask them to be precise about what they want, or to give/show you an example that shows precisely what they want.
I need to create a certificate with the following extentions:
Is it possible ... the addtext argument must be used? [You mean -addext in req]
These are not extensions. X.509 certificates have name fields containing attributes, which are one thing and the thing you want, and separately (but similarly identified by OIDs) they have extensions, which are a completely different thing that you do NOT want. Or at least not for this purpose; as you proceed you may find you also want extensions for something completely different that the specifier 'forgot'.
