Just drag one simple textbox from the toolbox and just add attributes like the below screen shot.
Then, as we know, just write the required functions inside the Script tag as below:
That's it, now this function will be triggered on the client side and you will get an output like the below screen shot.
While the cursor is on the Bill No. field:
Once the cursor moves away from the Bill No. field:
Hope this was useful! Kindly let me know your feedback or thoughts.