Message board should be safe if you can see a padlock Icon in the browser address bar similar to this:
re: "Where do you send login credentials to Hired Freelancers? in the Message board?"
Honestly? That's what clients usually do with me.
Is it "safe"?
...I don't know. "Technically" it isn't totally secure. Obviously Upwork has access to it.
For most purposes, it's probably fine.
You're creating a website to promote your hand-crafted Panda bear figurines made from driftwood? I wouldn't worry about it.
You're setting up a server to house the NOC list which identifies all of our undercover operatives active in enemy countries?
Maybe use a different means to send login credentials.