Keep Coding

I've configured my google app engine with a custom url. I added a new cname entry in my dns provided which look like this:
app CNAME ghs.googlehosted.com.
Every time I enter the url app.website.com, I get automatically redirected to the preview url https://20161211t143424-dot-dfsf-151819.appspot-preview.com. This is really annoying, how can I use my app in my custom url? Do I need to use a https?
You can see the behaviour here : http://app.e-writer.io/

This seems to be the default behaviour for the (new) flexible environment these days. From Domain update (emphasis mine):
Traffic is now served from the appspot-preview.com domain instead of
the appspot.com domain. All traffic from the App Engine flexible
environment will be automatically redirected to the new domain. All
App Engine Standard traffic will continue to be through appspot.com.
When the App Engine flexible environment is generally available (GA),
users will be able to use the appspot.com or appspot-preview.com
domains for routing traffic.
For users who want to use the latest App Engine Flexible release with
a custom domain, please fill out this form to contact us.
Looks like you have to contact them using the above referenced form.
You might be able to switch to the previous flexible env version (i.e. using vm:true in the app.yaml). But documentation for that may be scarce now and I'm not even sure if it'll allow the custom domain to work (I didn't use any of the flex env yet, so this is just a theory).