Hi,
What's wrong with the expression "Please send urgently about"?
Just about everything. We don't say "Please send urgently". You need a noun between "send" and "urgently". For example, "Please send your response urgently". Even that isn't particularly natural!
Also, you need that noun in order to follow it with "about". Even without "urgently", "Please send about ..." is ungrammatical.
The urgency comes from the person making the request, not the sender, so I'd say "I/We urgently need your response regarding the proposal. Please reply by 3pm today".