The ESP module can read a SI7021/HTU21D temperature/Humidity sensor using the I2C protocol. (The HTU21D Sensor does have the same specification and communications protocol and even the same I2C address so it is drop-in replacements for the SI7021)
The sensor needs to be connected to a the I2C pins on the ESP module. These pins can be configured on the hardware tab.
Use the device tab on the ESP Easy webinterface and create a new task by editing one of the available tasks. Select "Temp + Hum SI7021" from the dropdown box.
Enter the IDX found in the Domoticz device page. That should be all.