ACS_SendString(int scriptnum, str s) ACS_NamedSendString(str scriptname, str s)

These functions will launch the specified clientside script and send the string to it. For example:

ACS_SendString(123, "Hello there");
ACS_NamedSendString("myscript", "A test string");

To receive a string, the receiving script must be like this:

script 123 (int a, int b, int c, int d) CLIENTSIDE
{
        // Do nothing if the string hasn't been fully received.
	if (!ReceiveString(a, b, c, d))
		terminate;
	
        // The string has been fully received. It can now be retrieved using GetReceivedString().
	Log(s:"Received string: ", s:GetReceivedString());
}

Use GetReceivedString() to retrieve the string that has just been received.