How to prepopulate the textbox once I clicked the textbox. Initially Textbox will be empty, only after I clicked it should get populated.
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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. orIf 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.