Chat is supported with the Windows and Mac versions of the Splashtop Business app (purple icon).

When already in a remote session

In-session chat is available to users of:

Splashtop Business Access Pro
Splashtop Remote Support (Plus/Premium)
Splashtop On-Demand Support

Click in the chat field of the toolbar and start typing.
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User at the remote computer will see the chat window pop up. This is where the user would type. It's also where you see what the user types.

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When you're not in a remote session

(Requires version 3.2.4.0 or newer of both Splashtop streamer and Splashtop Business app)
Off-session chat is available to users of:

Splashtop Business Access Pro
Splashtop Remote Support (Plus/Premium)
SOS+10 & Unlimited

Launch the Splashtop Business app. Click on the chat icon next to the computer you want to start a chat session with.

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Notes

Chat only works when the remote computer is logged in. (Chat window is not visible on the Windows login screen.)
The name shown in chat is your Display Name or Splashtop ID (if Display Name is not set).
Chat is only supported from Windows and Mac versions of the Splashtop Business app. It's not supported in the iOS and Android versions of Splashtop Business app.