How can i add a checkbox in the form in the log record?
There are two approaches, depending on what you need…
You can enable the Field UI module which will expose an admin UI for adding fields to specific log types.
Or, if you want to add something to ALL log types, then you could use
hook_form_alter() to add it.
(This is all assuming you are doing this in farmOS 1.x - which I think is true. farmOS 2.x makes this process a bit easier too.)