I'm trying to set one of my shares (misc) to a security level of private and then giving just one of my users read/write access. When I browse to the share from Windows Explorer in Windows 10 (\\server\misc) I get prompted for login creds as expected, but no combination of username/password will work. I tried just username and password, server\username and password and username@server and password. Is there a special way I need to do this to get it to work from windows?

Windows will only allow you to have one login per network connection. If you have already accessed your server with a different login than the one you are trying to use you will get exactly what you have described. Go to Control Panel - Credential Manager and delete all credentials for your unRAID server. See here for more

Thank you, my searching skills were lacking. This is the info I needed to figure out how to fix the problem. I ended up creating a user in unRAID that matched my windows username.