You may denote a prerelease version. Or not. But you can't just append any crap you like and call it the prerelease version.

The wording is ambiguous, but the BNF later in the spec [1] agrees with your interpretation. Valid version numbers are three numbers separated by dots, followed by either a minus and dot-separated pre-release versions; or a plus and dot-separated build identifiers.

1: https://semver.org/#backusnaur-form-grammar-for-valid-semver...

No, that is a misreading. The "MAY" indicates that the prerelease identifier itself is optional. However, if you do append one, it must include a leading hyphen.