Comment 12 for bug 1764735

It should be available as --candidate, so you should be able to:
 snap refresh juju --classic --candidate

(We automatically build candidate stable to 'candidate', and we build the develop branch into --edge)

$ snap info juju
name: juju
summary: juju client
publisher: canonical
contact: http://jujucharms.com
license: Other Open Source
description: |
  Through the use of charms, juju provides you with shareable, re-usable, and repeatable expressions of
  devops best practices.
snap-id: e2CPHpB1fUxcKtCyJTsm5t3hN9axJ0yj
channels:
  stable: 2.3.7 (4152) 54MB classic
  candidate: 2.3.8+2.3-6caee25 (4311) 55MB classic
  beta: 2.4-beta2 (4242) 55MB classic
  edge: 2.4-rc1+develop-192d958 (4315) 63MB classic

If you used --edge, then you would get the 2.4 pre release.