'BackgroundModes' - system capability is not listing inside the 'project.pbxproj' file of an iOS project

I am trying to integrate the Background fetch feature inside a Flutter project using Workmanager plugin. So I enabled the background fetch feature under 'Background Modes' of my iOS project Capability. But the following block is not added to project.pbxproj file. As I understood, it should be automatically added by XCode.

SystemCapabilities = {
                            com.apple.BackgroundModes = {
                                enabled = 1;
                            };
                        };

Currently, I am working with Xcode 11.3.1 and Swift 5.0. So how do I bring the above snippet in project.pbxproj file?. Or please help me if my understanding is wrong.