how to include a "extra launch script"

Hi, we've been previously using the slurmspawner here: https://github.com/mkgilbert/slurmspawner . We are getting close to having our config converted. Yet one thing remains. What is the equivalent of this:

c.SlurmSpawner.extra_launch_script = pjoin('/etc/jupyterhub/extra_launch_script')

Was thinking maybe this was:

c.BatchSpawnerBase.req_prologue = pjoin('/etc/jupyterhub/extra_launch_script')

But in the logs I see:

[W 2018-06-20 16:19:50.830 JupyterHub configurable:168] Config option `req_prologue` not recognized by `SlurmSpawner`. Did you mean one of: `req_cluster, req_nprocs, req_runtime`?

We use the extra launch script to prep the environment for the jupyter job.