As described here, the stm32duino platform core has the support of Generic STM32F103Rx but I can’t find the board ID to use them while in Arduino IDE you can find them in the menu, what’s the problem ?
You must reference valid PlatformIO board names, not STM32Duino internal variant names. Those are referenced in the PlatformIO board files as variant. Search for boards in PlatformIO Registry.
E.g., the STM32F103RB is at STM32F103RB (20k RAM. 128k Flash) — PlatformIO latest documentation with a name of genericSTM32F103RB.
I know that but the problem is that, when I use genericSTM32F103RB the PIO switches to maple core instead of STM32Duino which I need to compile the code with, while as described here the STM32Duino also has implemented the MCUs as well and you can easily chose them in Arduino IDE so what’s the problem ?
What happens when you read the documentation for the platformio stm32 platform support, and use board_build.core = stm32 in your platformio.ini to switch Arduino cores? 
well, there just mentioned how to switch to maple core not how to switch to stm32 core 
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Well, to be pedantic, it mentions how to changes cores generally, and then gives an example for the maple core. But, point taken, it doesn’t give an example for switching to the stm32duino core. btw, I probably should have listed it as ststm32, but AFAIK, anything that isn’t ‘maple’ should work.