Odoo 9.0 community edition.

When there is a many2one field in an odoo from it is possible to select "create and edit" from the dropdown menu when populating this field in edit mode. This brings up a window which allows you to create a record. I know that you can also provide a "context" such that some of the fields of the new record are pre-populated for you.

I'd like to have a button with a server action that let's me do some calculations on what the context should be and then bring up this same window (new window). In particular this is about creating a contact from a sales opportunity. I was not able to find the window action to call. I tried the window action called Contacts, and this brings up a contact edit window, although there is no Save button at the bottom.