it looks like they are able to reference mscorlib https://github.com/FStarLang/FStar/blob/997b1f20f35142e65032... and F# with these externs https://github.com/FStarLang/FStar/blob/997b1f20f35142e65032...
It appears that the correct way is to create a .fsi file.
A few examples: https://github.com/FStarLang/FStar/blob/4e4162f736195b2031ba...
https://github.com/FStarLang/FStar/blob/4e4162f736195b2031ba...
https://github.com/FStarLang/FStar/blob/4e4162f736195b2031ba...
https://github.com/FStarLang/FStar/blob/4e4162f736195b2031ba...
https://github.com/FStarLang/FStar/blob/4e4162f736195b2031ba...