This would only be possible if the hardware of the USART in your particular stm32 supported this.
And it might be that the library chooses to hide such a feature in order to present a common interface independent of whether such a feature were present or not.
I'm not aware of any that do support inversion of the RTS polarity. But then I don't know the entire family so it might be that your one can do this in hardware.
On the other hand, there's nothing stopping you doing this in software. You'd need to program RTS as a GPIO input (rather than alternate-function), and enable interrupts on change to high. Then you'd could run the USART to transfers by interrupt (rather than DMA), putting enough intelligence in the USART interrupt handler to stop TX (and disable the TXEIE / TCIE interrupts) when RTS is low; re-enabling them when you get a rising-edge-interrupt on RTS.
This is all possible. But you need much greater understanding of the peripheral itself as described in the Reference Manual; things that the ST/HAL libraries hide from you (which gets you started much more quickly but might not let you do precisely what you want to).
Hope this helps,
Danish
