This requires no additional components; just 3.3V, Gnd, and the signal connected to PB1

You /can/ do this with the on-chip bootloader, though its a little bit tricky and must be done over serial.  See application note AN3155 for details.  Short story is you will need to hook a FTDI-232 to PA9 and PA10 and move the BOOT0 jumper and use some software you download from ST.  The newer "STM32CubeProg" supports various OSs.
https://www.st.com/en/development-tools/stm32cubeprog.html#get-software
I instead personally prefer using the ST-Link v2, clones of which can be had for about $2 an Ali Express, etc.

Freshly flashed to an erased device, the system supports a command line interface ('monitor') on the USB as a CDC serial port.  You can plug it into your computer and connect via a terminal program to change settings.  The default printer model is an EM-205.

The printer requires a lot of power and will require an external supply of 5V 4A.  But the BluePill is designed to run only from USB power, and it is not isolated.  For those that want to support stand alone operation (i.e. just power supply) while also supporting USB connection, you will need to install two power steering diodes to prevent the power supply from entering the USB.  I use two schottky diodes: one between VBus and the internal 5V rail, and one from the 5V pin on the board into the internal 5V rail.  That way you can freely operate both ways, safely connecting to usb whenever you want.  But this does require trace cutting.
If you do those board modifications, you can have the board powered from the external supply as well for standalone operation.

The printer must support 3.3V ttl serial ONLY -- these pins are /not/ 5V tolerant.
The printer TX is connected to the board RX at PA3.
The printer RX is connected to the board TX at PA2.

There is about a 10 sec delay on cold boot because the board has to wait for the printer to have finished it's own boot process, which can be slow on some models.
Wait until the green light extinguishes, and it should be ready to receive IR for printing.
There are some progress messages emitted to the monitor port.