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It would help our users to be able to provide placeholder text on a textbox, which shows them an example of a value they might enter. Currently there does not seem to be an option to provide this to the form element.
You can set a default value to textboxes using the "defaultValue" parameter.
For example: Input.ShowTextBox("Textbox Name", "textbox_label", defaultValue: "Modified default value");
For more information please see the ShowTextBox developer documentation
I'm referring to placeholder text rather than an actual value of the form field.
Personally I use the helpMessage parameter but I agree - placeholder text is a very common UX element that does the job more efficiently.