How to post a form to django site, securely and from another site
I have a django app which need to receive post requests from another
  site using html+javascript.

You don't have to ! You can build an API instead ;)
You create an API call - small site calls the API of main site. In that situation, the form is handled by a view in small site and the API is called via the server. Check out Django REST Framework. 
Note: That solution wouldn't stop you from using AJAX but it would avoid cross domain issues.
I have a django app which need to receive post requests from  another site using html+javascript.
For example i have mydjangosite.com and  smallhtmlsite.com
What i want is : user visit smallhtmlsite.com and fill a form, then he pushing submit button and mydjangosite.com receive request and create objects(form saving models actually).
So i will have a view which will handle this requests. But how can these been done securely?