We can add an open URL action in iPhone application by the following line of code.

[[UIApplication sharedApplication] openURL:[NSURL URLWithString:@"https://ykyuen.wordpress.com"]];

please note that it will close the application and open the URL in browser.

And if you want to have a direct link to Apple appstore, you can get the URL from iTunes Link Maker.

Reference:

• url link in iphone application
• iTunes Link Maker
• iTunes Link Maker FAQ