If your PLC has an internet conntection, you can use the HTTP Rest API for write data or you can write with MQTT publish via the broker at mqtt3.thingspeak.com.
If not, you can use a low cost internet capable microprocessor board to communicate the PLC data to ThingSpeak. I might suggest and arduino nano 33 iot, or ESP32. You can program them in C to read data from the PLC via i2c, serial, or SPI then write to ThingSpeak. A word of caution that the data rate allowed at ThingSpeak is reletively low compared to typical PLC speeds. You may want to consider preprocessing on your device to reduce the throghput.
