You want to make a visible background behind the text in a text field.
Set the text field's background property to true. Additionally, you can change the color of the background by setting the object's backgroundColor property.
Text fields lack a visible background by default (in other words, the background is transparent). However, you can create a background for a text field by setting the background property to true for that object:
myTextField.background = true;
By default, the background for a text field is white (if made visible). You can, however, assign the background color by setting the value of the object's backgroundColor property, which accepts a hex RGB value corresponding to the desired color:
myTextField.backgroundColor = 0x00FFFF; // Set the background to light blue.