Use a user script that injects cookies

WKWebView_Add_UserScript

WKUserScript *userScript = [[WKUserScript alloc] initWithSource:@"document.cookie = 'CookieKey=CookieValue';"
                                                  injectionTime:WKUserScriptInjectionTimeAtDocumentStart
                                               forMainFrameOnly:NO];

WKUserContentController *userContentController = [[WKUserContentController alloc] init];
[userContentController addUserScript:userScript];

WKWebViewConfiguration *configuration = [[WKWebViewConfiguration alloc] init];
configuration.userContentController = userContentController;

WKWebView *webView = [[WKWebView alloc] initWithFrame:CGRectZero configuration:configuration];