The 'es' is implied. When you say 'I know' you really are implying the specific item that you know. In German (and French and some other languages) you must spell out the missing 'es'. On its own "I know" does not mean anything. What do you know? We just imply 'it' in English and that 'it' can mean many things depending on the context.
It isn't simply stating that the speaker doesn't know an answer.
It is also implying that the person they are speaking with doesn't know and they are trying to appease their emotions for not knowing.
Hey honey, how do you spell acquiesce?
I don't know exactly myself.
1st person might feel stupid because they didn't know so the 2nd person attempts to alleviate that feeling with the addition of "myself".
"I do not know exactly myself" sounds very unnatural. It would sound far more natural to say ""I do not know myself, exactly", or "I do not know exactly", , or maybe "I do not know, myself"depending on what you want to emphasize. Still, the "myself" seems forced, as a way to practice "selbst" in the German sentence.
I find it helpful to inflect my voice as if I were in a real conversation. Ex: I do not know it exactly MYSELF, but.. blah blah... here's where you can find out. This helps commit rarely spoken sentences to memory. In the future when you need to emphasize or inflect a word like selbst with voice, your memory will recall the. In other words, when we use a word like myself, it just so happens we are probably inflecting the voice in a rare sentence.. Hope this helps...