phrases – Should I use “at soonest” or “as soonest”? – English Language & Usage Stack Exchange

Soonest is used to say ‘as quickly as is reasonable’, it is the superlative form of the word of soon. (soon -> sooner -> soonest). “As soon as possible” is what you should use:

Please let us know as soon as possible (or as early as possible) if you can help us.

Usage in a response:

I don’t know for sure, the soonest I can know is later this evening.

OR

The soonest possible time I can let you know is at 5 pm.

You may hear someone say to someone else “see you soonest” but it’s colloquial and what they mean is “see you in a flash” or “see you as soon as I can” or “see you very soon”.