I mean default allow discuss when add in Plone 5
The default value for the 'Allow Discussion' setting for contenttypes is False, so no.
Though you can go can to the content types control panel and enable it fairly quickly.
Don't forget to enable comments globally too: http://docs.plone.org/adapt-and-extend/config/discussion.html
If you develop custom contenttypes on the filesystem, you'll make a profile/default/types/[portal_type id].xml file in which you can set this.
The easiest way is to enable this TTW, export the types tool in portal_setup and put that in your addon's default profile. If you're making a generic reusable addon, please try to only include the parts of the xml that contain the bits you need changing. ie, just the allow_discussion snippets.
Is that possible to auto enable the allow discussion when a item is added instead need to tick every time?