Rules in Requestly helps you modify a specific network requests or responses.
For this you need to connect the app, once you see the network traffic, follow below steps to create the rule.
Right-click on any of the request to open a context-menu.
Now from context-menu click on the appropriate rule type for your usecase. Learn about different types of rules from here.
It will open the rule editor, in that fill the details for the rule and click on Create rule on top-right in the editor modal.
Now when the same request is triggered again, you can see that the rule which you created is applied on it. You can verify this by clicking on request which will be highlighted (indicates rule applied), below in request pane you can see the modified request.