pOH in case of NH3

If NH3 has no OH-, how can we determine its pOH or pH?

Actually we determine the pH and pOH of the solution (solvent water). By adding an acid or a base to water,the [H+] or also the [OH-], of water is decreased or increased. And we calculate this change called pH or pOH. eg if we add NH3 to water, it will decrease its [H+] by forming NH4+. So the solution will become basic just like adding NaOH which decreases the H+ concentration by forming H2O. And now the water, the solution will be basic.

  H2O--------H+  +  OH-  (neutral)
  Adding NH3
  NH3:  +   H+ (H2O) --------  HN4+ (basic)
   Adding NaOH
  OH-  +    H+ (H2O) --------   H2O (basic)