UIApplication and its methods are not available in iOS extensions, such as Share Extensions or Custom Keyboards. However, you can hack your way to, say, openURL if you really need to…

UIApplication+Hacks.swift

import UIKit

extension UIApplication {

    public static func hacked_sharedApplication() -> UIApplication {
        let selector = NSSelectorFromString("sharedApplication")
        guard UIApplication.responds(to: selector) else {
            fatalError("UIApplication.sharedApplication(): `UIApplication` does not respond to selector `sharedApplication`.")
        }

        guard let unmanagedSharedApplication = UIApplication.perform(selector) else {
            fatalError("UIApplication.sharedApplication(): `UIApplication.sharedApplication()` returned `nil`.")
        }

        guard let sharedApplication = unmanagedSharedApplication.takeUnretainedValue() as? UIApplication else {
            fatalError("UIApplication.sharedApplication(): `UIApplication.sharedApplication()` returned not `UIApplication` instance.")
        }

        return sharedApplication
    }

    @discardableResult
    public func hacked_openURL(url: URL) -> Bool {
        let selector = NSSelectorFromString("openURL:")
        if InvocationHelper().perform(selector, with: url, target: self) {
            return true
        } else {
            return false
        }
    }
    
    public func hacked_canOpenURL(_ url: URL) -> Bool {
        let selector = NSSelectorFromString("canOpenURL:")
        if InvocationHelper().perform(selector, with: url, target: self) {
            return true
        } else {
            return false
        }
    }

}