send 9 bits via hserout

[helloo]

Hello Folks,
I'd like to build my own serial communication protocol with rs485. Therefore I have to send the data in 9 bit mode.
Example:
first byte + 9th bit set -> that's the adress of the slave
all other bytes are send without the 9th bit set to indicate they're no adress bytes.

Question: How can I set the 9th bit manually?

my code so far:

DEFINE HSER_BAUD 19200        'Set Baud rate to 19200bps
DEFINE HSER_BITS 8        'Set to 8 bit mode
DEFINE HSER_EVEN 1        'Set Even Parity
DEFINE HSER_CLROERR 1         'Clear overflow error automatically

... some other code ...

HSEROUT [Slaveadress,Datalength,Databyte,Databyte ...until datalength reached...]

I will change the line:

DEFINE HSER_BITS 8        'Set to 8 bit mode

into:

DEFINE HSER_BITS 9        'Set to 9 bit mode

But how can I set the 9th bit????

Regards.