While ignoring commands preceded by a space character is absolutely one of my very favorite shell features, it usually isn't enabled by default (YMMV).
HISTCONTROL=ignorespace # bash
setopt hist_ignore_space # zsh
These go into ~/.bashrc or ~/.zshrc, respectively. zsh is really smart about this and always enables you to retrieve the immediate previous line with arrow-up, no matter whether it's got preceding whitespace or not.
+1 to prefixing with a space. hiSHtory supports this out of the box so that anything prefixed with a space isn't recorded (even if your shell doesn't do that by default!). And if you do ever mess up, `hishtory redact` can be used to delete history entries.
Not that it probably would have helped in this situation, but maybe useful for others.
Usually if I have to enter secrets in my shell, I do something like this in order to save the useful stuff to history but hiding the secrets:
- ` export PASSWORD=hunter2` (notice the space before "export")
- `service-cli login username $PASSWORD` (no space before, gets saved to shell history)