link next to field to add referenced entity for site editor EDIT: I want link button in an edit form of a content type like 'store' that says 'add store section'. This link/button should directly go to  the edit form that adds a new 'store section' which is pre-configure by me as entity reference. 
This link would be next to a drop-down list of already available 'store sections'.  

First step would be that the link needs to go to the node/add form of contenttypeB. 
I've heard as a hint that hook_field() functions are what I need.
Something would have to get the form id somehow and this is where I'm confused, I'm wondering if I need to address the contenttypeB nodeforms hardcoded into my .module file? Or do I just need to start doing this by looping through all the fields of the content types and checking whether those fields are an entity reference and then apply it?

I'm still very scared of module development and php as I can stare at code for too long. I'm sort of getting the hook concept a little bit now. Things like hook_men() and then how it goes to the callback function I sortof get. Or when it can go to drup_get_form() and you fill in the function to build a form, I can dig that. 

Just to say what level I'm at.
This could be done without a custom module, by using the Markup module.

However, if you are looking for the ability to add a new ContentTypeB that is immediately referenced, the References Dialog module can do this.