You do not need to save the number first. The chat lands in the manual queue when you confirm.
- Click the chat icon at the top left, next to the contact list.
- Select the channel and the desk.
- Pick the country code and type the customer number.
- Choose the opener: quick reply, your own text, or a whisper.
- Click OK. The chat goes to manual inbox.
The number can be missing from contacts. The queue still takes it.