The function HAL_UART_Transmit() does not care about character formatting, it just send a bunch of data with 7/8/9 bits each. What you have to do now is to sort the bytes of the character set and send it one after the other so that they can be received by the recipient as your expected Extended ASCII set.
Of course you have to know what has to be transmitted, i.e. exactly how your character set is structured, because there is not only one definition of Extended ASCII, see here.
Regards
/Peter
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