Customer has windows server on private range behind Draytek and customers - end customer wants to access same server(Windows server) from public. as per request I had added port direction on port 80  for both public & private ports filed in port redirection and Filters allowed from WAN->LAN with source as Any and destination as Windows server and ports as 80 & 81. But it is not working as expected. 

Please help on suggestions to setup this correctly. 
 

So there is a service running on the Windows Server on Ports 80 and 81? And the customer is trying to get their end customer to connect to that service. What service is it, in case there are specific details we need to know?

You should lock down the firewall rules so that only the end customers IP address can connect to an internal Windows Server, especially on port 80. That is assuming that the end customer is just one company.

If this is a public facing service for many customers, there are massive risks to what your customer is attempting.

Yes it is public, but it is allowed specific host on Firewalls.
End customer is still not able to access windows server as expected.
I had given public & private port as 80 on POrt Redirection configurations. Anything else I am i missing here ?

Windows Firewall?

I think there needs to be some clarification on what access is required.
Port 80 is usually http so that'd require a web server running on the windows server; what that windows server presented is up to the user but it wouldn't (normally) allow them to access the files on the machine, log in as different users etc. so do they mean they want to access it using something like remote desktop i.e. different users could log in as themselves, see the desktop pertinent to them, edit their files - run Microsoft Word for example?

From an elevated windows command prompt you can use the netstat command.
netstat -af is a good start - look for the lines with "LISTENING" which will show you what server ports are in use (i.e. have a process waiting to receive data/requests)
netstat -afb will show you the process used which may be useful.

*BUT* I'd agree with Bob Bob - be very wary indeed. I'm not sure (and absolutely no disrespect meant - I'm erring on the side of caution rather than being wild west) the requirement is fully understood and that could potentially result in an insecure environment. Sorry if you've been necessarily paraphrasing.

Another quite useful (but far from comprehensive) is the GRC ShieldsUp tool - https://www.grc.com/shieldsup
Common Ports and All Service Ports (which I'd argue doesn't do exactly that) can be useful. Before you do anything (so you might want to revert any previous changes you've made to firewall(s) etc.), run that, see what it shows and then go from there. As I said, a quick scan, certainly *not* comprehensive security scan.

Oh and one last thing - on my Draytek router, there's an option Firewall / Diagnose which should allow you to do a bit of testing; I'd devise the test before you actually made any changes if I were you but that's my preference.