You only need to specify a version string that uses SemVer format (e.g. 1.0-beta) instead of the usual format (e.g. 1.0) and NuGet will automatically treat it as a prerelease package.
“As of NuGet 1.6, NuGet supports the creation of prerelease packages by specifying a prerelease string in the version number according to the Semantic Versioning (SemVer) specification.”
See NuGetDocs – Prerelease Versions