RouterOS does not have a random function. Many has tried to make script to make random text, but all seems to be flawed.
But this should do, since it gets the string from an external site:
{
/tool fetch url="https://www.random.org/passwords/\?num=1&len=8&format=plain&rnd=new" keep-result=yes dst-path="pass.txt"
:local pass [/file get pass.txt contents]
:put $pass
}
EDIT
No need to use a file. This should do:
{
local pass ([/tool fetch url="https://www.random.org/passwords/\?num=1&len=20&format=plain&rnd=new" output=user as-value]->"data")
:put $pass
}