You can use the write_table! function from
https://github.com/lungben/TableIO.jl
It implements the COPY FROM STDIN functionality using CSV.jl and LibPQ.jl.
https://github.com/lungben/TableIO.jl/blob/ce1f205053cee0270ae2b53c0787d576f70e0c4e/src/postgresql.jl#L32