class CustomWebView: WKWebView {
    var accessoryView: UIView?
    override var inputAccessoryView: UIView? {
        return accessoryView
    }
}

class ViewController: UIViewController, WKNavigationDelegate, WKUIDelegate, WKScriptMessageHandler,ASAuthorizationControllerDelegate {
...
public var wkWebView: CustomWebView? = nil
...

self.wkWebView = CustomWebView.init(frame: CGRect.init(x: 0, y: 0, width: self.view.frame.size.width, height: self.view.frame.size.height), configuration: self.config!)

CustomWebView를 선언하고 해당 class로 바꾸면 된다.

How do I handle return key on an iOS web view native keyboard?