The it forms and there is, there are forms are used in English only when the subject/actor noun of the sentence can’t do any action or else

change of action volitionally at all.

Examples

There is a gas station on the corner of Sixth and Elm.

It is true the Earth moves around the sun.

In your sentence, (we) need a response, ask for a response, and tell someone we would appreciate a response.

The other person is being moved, hopefully, to make that response.

You can’t order that person to give it, so the imperative form is not useful; but presumably you expect the person to come across with one.

There is no one answer to this question; more than one sentence would have a good effect, perhaps.

But in real space-time, (we) are asking for a response which they will supply–or not–on time; and then (we) are specifying that if the client grants one, it is required to arrive to (us) no later than tomorrow at closing time.

Forget the appreciation. You have nothing to appreciate until the action has been named that will earn it.

Begin with the action is my advice. In reality, tell the client who is doing what to whom.

“If you could give us your response today or tomorrow,

we would greatly appreciate your action/kindness/consideration/help.”

soonest in this case would refer to as soon as possible in the future which can refer to today, tomorrow or anytime in the future.