Hi Anupama,
Under the properties of the Texbox that you have used for PS Number, there will be section called Events.
Use on change and call a microflow.
And In the microflow, retrieve the Entity that you have used to store the Patient name and based on the user entered PS Number Filter the Retrieved Data and Use a change Object (Object should be your Entity), select the memeber PSName and value will be the PSName from Retrieved data based on PS Number.
Hope so this helps you!
Thanks,
Amulya Hc