set send-prompt

SET SEND-PROMPT [string]  -  define the prompt for Send Mode
Usually:        "Send>"

The SEND-PROMPT variable defines the prompt for Send Mode.  Another
  common prompt is "S>".

For more help, type "help" and one of the following:
(at any prompt:)  save-init   set   show
                  set top-level-prompt   set read-prompt