ct supports generating completion scripts for Bash, Fish, Zsh, and PowerShell. See ct help completions for full details, but the gist is as simple as using one of the following:

# Bash
$ ct completions bash > /etc/bash_completion.d/ct.bash-completion

# Bash (macOS/Homebrew)
$ ct completions bash > $(brew --prefix)/etc/bash_completion.d/ct.bash-completion

# Fish
$ ct completions fish > ~/.config/fish/completions/ct.fish

# Zsh
$ ct completions zsh > ~/.zfunc/_ct

# PowerShell v5.0+
$ ct completions powershell >> $PROFILE.CurrentUserCurrentHost
# or
$ ct completions powershell | Out-String | Invoke-Expression

Note: you may need to restart your shell in order for the changes to take effect.

For zsh, you must then add the following line in your ~/.zshrc before compinit:

fpath+=~/.zfunc