How to make textbox with underline?

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Just add one text box and set width to fill… automatically you will see such line

Yes, i know but how to change the color, because when i change color , the line invisible

AFAIK, The line color is same as your accent color so if you want to change the underline color then change the accent color of the app and make sure that your textbox background color should not be white or none. You can leave it to default color.
There is another way to do this, Just put a label below the textbox and set the label bg color to whatever you want and set the text to none. Afterall, the result would not be exact but you can manage it by setting the margins of the label.

Ok Note. Thanks