I have a content type with a paragraph entity reference field and every time a user is going to add a node of this type I want to have the paragraph field already open and prefilled. I've tried with hook_form_alter, but it's not opening the paragraph form with prefilled values.
In the form display for that content type you can tell the paragraph field to have a default paragraph selected. So when new nodes are created it will appear there
Then on your actually paragraph you'd need to set the default values for each field it contains (Just by editing those fields on the paragraph and setting the defaults)