This worked for me recently (julia v 1.5.1, macOS 10.14):
using IJulia
notebook(dir="/path/to/directory/with/my/notebook",detached=true)
One thing I noticed is starting jupyter in the background, julia doesn't give you a link to where to open the notebook in the browser.
I was able to open http://localhost:8888/
in my browser to find the notebook, although I thought there was a URL token needed. This link in the browser also worked:
file:///$HOME/Library/Jupyter/runtime/nbserver-12912-open.html
I got this from the REPL documentation:
?notebook
#search: notebook
# notebook(; dir=homedir(), detached=false)
# ..etc