When create a new mailbox or any other command which needs input password, you may create this variable which can be used as the value of the parameter "-Password".
[PS] D:/Windows/System32>$pwd = Read-Host "asd" -AsSecureString
asd: *******
[PS] D:/Windows/System32>New-Mailbox -Name user1 -Database "First Storage Group/Mailbox Database" -UserPrincipalName user1@irkgai-dom.extest.microsoft.com -Password $pwd
Name Alias ServerName ProhibitSendQuota
---- ----- ---------- ---------------
user1 user1 exch-i-856 unlimited