I don't believe Citect has a native driver for this, so you will likely need to involve an external application also.
I'd recommend investigating node-red, here's google result 1 for 'node-red esp8266 tutorial'
Once you get the data into node-red, then you should be able to use OPC (server), or even perhaps ModbusTCP (server) or another such interface to present your data for consumption by Citect.
Citect itself would then be an OPC client / ModbusTCP client etc.
The ESP8266 can be programmed using the Arduino framework (https://www.arduino.cc/en/Main/Software), so you can find Modbus library for this module:
Then use the standard Modnet driver or OFS driver from Citect SCADA to connect to the ESP8266.
I have managed to make the connection with citect with modbus code on my esp sending temperature data .
if someone wants I will post the code for esp here.
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