How to prepopulate the textbox once I clicked the textbox.

Simple answer - You cannot do that in Appian. 
Workaround - You can have a button adjacent to textBox that can actually populate the value in the textbox. 
or
If the value depends on some other fields on the same screen, then you can generate the value on the go and only show it in the textbox when the complete value has been generated. 

What value do you want to populate? On the button click you can do it or if you don't want to do it an extra click then on the save into of any other component you can save the value into the variable and use that variable to show the value into the field.

Harshit's simple answer above is basically correct.  But the real question here is, what's your use case?  i.e. what actual functionality are you hoping to accomplish, and why?

Use case is simple.....I have a Textbox where I write some number, once I move out(UNFOCUS) of the textbox the text will be masked. after if I come back to the same textbox I need that text to be unmasked.

You can use the "masked" parameter on the a!textField() component.  You *can* swap this value on and off depending on how other values change on the form, but there's nothing currently that can cause masking to automatically turn on or off depending on the field's focus status.

Thank you for your information.