Ios – WKWebView cookie storage location

Yes, you can do this. First, in applicationDidBecomeActive add this line

[[NSHTTPCookieStorage sharedHTTPCookieStorage] setCookieAcceptPolicy:NSHTTPCookieAcceptPolicyAlways];

The cookieAcceptPolicy is shared across apps and can be changed without your knowledge, so you want to be sure you have the accept policy you need every time your app is running.

Then, to set the cookie:

NSMutableDictionary *cookieProperties = [NSMutableDictionary dictionary];
[cookieProperties setObject:@"testCookie" forKey:NSHTTPCookieName];
[cookieProperties setObject:@"someValue123456" forKey:NSHTTPCookieValue];
[cookieProperties setObject:@"www.example.com" forKey:NSHTTPCookieDomain];
[cookieProperties setObject:@"www.example.com" forKey:NSHTTPCookieOriginURL];
[cookieProperties setObject:@"/" forKey:NSHTTPCookiePath];
[cookieProperties setObject:@"0" forKey:NSHTTPCookieVersion];

// set expiration to one month from now or any NSDate of your choosing
// this makes the cookie sessionless and it will persist across web sessions and app launches
/// if you want the cookie to be destroyed when your app exits, don't set this
[cookieProperties setObject:[[NSDate date] dateByAddingTimeInterval:2629743] forKey:NSHTTPCookieExpires];

NSHTTPCookie *cookie = [NSHTTPCookie cookieWithProperties:cookieProperties];
[[NSHTTPCookieStorage sharedHTTPCookieStorage] setCookie:cookie];

This cookie has the name testCookie and value someValue123456 and will be sent with any http request to www.example.com.

For one big caveat to setting cookies, please see my question here!

NSHTTPCookieStorage state not saved on app exit. Any definitive knowledge/documentation out there?

This is actually a tough one because there's a) some bug that's still not solved by Apple (I think) and b) depends on what cookies you want, I think.

I wasn't able to test this now, but I can give you some pointers:

1. Getting cookies from NSHTTPCookieStorage.sharedHTTPCookieStorage(). This one seems buggy, apparently the cookies aren't immediately saved for NSHTTPCookieStorage to find them. People suggest to trigger a save by resetting the process pool, but I don't know whether that reliably works. You might want to try that out for yourself, though.
2. The process pool is not really what saves the cookies (though it defines whether they are shared as you correctly stated). The documentation says that's WKWebsiteDataStore, so I'd look that up. At least getting the cookies from there using fetchDataRecordsOfTypes:completionHandler: might be possible (not sure how to set them, though, and I assume you can't just save the store in user defaults for the same reason as for the process pool).
3. Should you manage to get the cookies you need (or rather their values), but not be able to restore them as I guess will be the case, look here (basically it shows how to simply prepare the httprequest with them already, relevant part: [request addValue:@"TeskCookieKey1=TeskCookieValue1;TeskCookieKey2=TeskCookieValue2;" forHTTPHeaderField:@"Cookie"]).
4. If all else fails, check this. I know providing just link only answers is not good, but I can't copy all that and just want to add it for completeness sake.

One last thing in general: I said that your success might also depend on the type of cookie. That's because this answer states that cookies set by the server are not accessible via NSHTTPCookieStorage. I don't know whether that's relevant to you (but I guess it is, since you're probably looking for a session, i.e. server-set cookie, correct?) and I don't know whether this means that the other methods fail as well.

If all else fails, you might consider saving the users credentials somewhere (keychain, for example) and reuse them on the next app start to auth automatically. This might not restore all session data, but considering the user quit the app that's maybe actually desirable? Also perhaps certain values can be caught and saved for later use using an injected script, like mentioned here (obviously not for setting them at start, but maybe retrieve them at some point. You need to know how the site works then, of course).

I hope that could at least point you towards some new directions solving the issue. It's not as trivial as it should be, it seems (then again, session cookies are kind of a security relevant thing, so maybe hiding them away from the App is a conscious design choice by Apple...).