It doesn’t exist from MicroSoft, but FreeBSD devotees have created it.
Long discussion here: https://github.com/dotnet/source-build/issues/1139
GitHub Page with scripts on how to build from source on a Linux machine for use on FreeBSD here: https://github.com/Thefrank/dotnet-freebsd-crossbuild/
And GitHub page with a link to a tar.gz of pre-compiled binaries: https://github.com/jasonpugsley/installer/wiki/.NET-5.0-Preview-3-for-FreeBSD
And the GutHub page on how to build the binaries yourself for the above tar.gz: https://github.com/jasonpugsley/installer/wiki/.NET-Core-3.1.103-for-FreeBSD