The TextMeshPro - Input Field is very similar to the UI Input Field. When added to the scene, you'll see it looks nearly identical, except that the placeholder text has a different font. The UI Input Field uses an Arial font by default, while the TextMeshPro - Input Field uses Liberation Sans.
To create a UI Input Field, select Create | UI | TextMeshPro - Input Field. By default, TextMeshPro - InputField GameObject has a child named Text Area, which has two children: a Placeholder and a Text object. You will observe that it is slightly different in setup than the UI Input Field.
The Text Area GameObject contains a Rect Transform component and a Rect Mask 2D component. The Text Area ensures that the text does not appear outside of a specified area, as shown by the highlighted area in the following image. If you wanted to change the size of...