When the pen cursor is moved into a text input box, the On-Screen Keyboard will pop up at the bottom of the screen, similar to the image below.

To stop this from happening. please follow the instructions below. 

[1] Click the [Start] menu of the computer.

[2] Then click to enter the [Settings] menu.

Click to enter the [Device] settings option.

[1] Click to select the [Pen & Windows Ink] option in the left menu bar.

[2] Find the [When I tap a text field with my pen, use handwriting to input text] option of [Handwriting], click to open the drop-down menu, and click to set it to [Only in tablet mode].

After setting, enter the text input box, the Touch Keyboard should not automatically pop up.