Hi Jagat,
As others mentioned, the nxgetAD you have defined in the DOMAIN-SPSADMINUI-... domain is the one that will be checked to authenticate the user & password you are introducing in the proxyui login page.
For this, there should be also a Policy where you specify which user/s will be allowed to access (even if the credentials are correct, so for example if you only wish an administrator account to login here).
So, you should already have a policy object (may be called POLICY-SPSADMINUI-... (etc)) where you need to define this. You may add a single entry or depending on the root/base DN you defined for the nxgetAD User Directory adding all may work for you. This depends on your needs, but the idea is to specify which user/s will be able to access the proxyui.
You should find this under the "Domain Policies" section (so in AdminUI, Policies > Domain menu)
This entry is added in the Users tab of the policy.
I hope this helps.
Albert F.