Can't find received_header_text in exim.config file

The first step is to find where the configuration file is located. When you run exim -bV it will print out the configuration file it is using. You can also see the current (and possibly the default) for that setting by running exim -bP received_header_text.

You must remember that the configuration file is likely strictly controlled by the DirectAdmin web admin, so you could really break your system if you're not careful. Chances are high though, that the config file shown from -bV above will be a dynamically generated file, and any manual changes you make will be overwritten the next time exim is restarted. I think you'll get better answers by pursuing this through the DirectAdmin forums as they know what requirements there might be to manually change the mail subsystem.