Update a field which doesn't exist

I’ve added a field to a content model(and i see it via web interface). But when I look at it via API it doesn’t exist, and if I try to write to it(( via entry.update()) I’m getting an error.

If I write in this field via interface (manually) and then do the update via the API(contentful-management) everything works fine.

So how I should do it? Add some new field via content model and then programmatically fill it out?