Requestly desktop and other similar tools run a proxy server on your local machine. You can get the proxy URL from Requestly and set it wherever required.
To use a proxy in iOS simulator you need to first set it to the system (choose Everywhere System Wide in Requestly) and then in your xcode project's scheme, define two variables http_proxy and https_proxy with value of the proxy URL and voilĂ .
To intercept HTTPS traffic you also need to install a root certificate.
In your simulator, launch Safari and navigate to http://requestly.io/ssl (Use http here, not https), tap Allow to download the Requestly Root Certificate and install it. Next, navigate to Settings > About > Certificate Trust Settings and enable RQProxy CA to mark it as trusted.
Edit: Their docs now has a detailed guide explaining step-by-step how to setup Requestly with iOS simulator