There is usually no way to automate the reading of capthca text fields unless there is a known algorithm from which the correct text can be determined. Captchas like this are specifically designed to prevent automated systems from access.

We would recommend disabling the capthca functionality for testing purposes, if this is possible. Alternatively if you have access to a DLL that creates the captcha then a DLL integration approach may be possible.

Further to this, many capthcas use the "reCaptcha" service which is a third party tool that uses a Google Web service to protect sites from bots and spammers.

With these it is impossible to decode the captcha image / text that is used as a challenge.  It is inadvisable to load test the reCaptcha service as this is not hosted by the application but rather by Google so you will effectively be load testing a Google service.

To conclude:

If you have access to how the capacha is generated, you can manually generate the code and provide via a CSV file or similar.

Or you can disable the capacha for the purpose of testing and simulate the time required for this by finding the average response time for reCAPTCHA using ping and use a sleep call to simulate that response time during load testing.