Xenforo plugin needs authentication

OK… So I am running an Xenforo plugin that uses XLink which does IP comparison to ensure that all API calls are coming from the same server. When using Cloudflare, this authentication method will fail, as my server accesses the remote platform API through a proxy or firewall, which masks the URL and will result in a negative check. As such, I am required to setup an API.

To do this, I added the foliowing line to my Xenforo config file:

$config['xpressAPIPassword'] = 'YOUR_API_PASSWORD';

And with that done, I was told:

Once you’ve set up your API password on your XenForo installation, you will additionally need to configure it on your remote platform. This approach is the same for all remote platforms. Open your remote platform configuration file and paste the following line at the bottom:

define('XLINK_API_PASSWORD', 'YOUR_API_PASSWORD');

Thing is, I don’t know where I need to do this within my Cloudflare dashboard.

Can anyone help me out?