Hi Cristian,
Thank you for your update.
From your update, we can see that you are not using Multiline mode. So your requirement for single line mode can be done by placing the TextBoxExt inside a panel and set the Border style to none. In this method, since we are using panel we can adjust the height of the panel instead of TextBoxExt. We have prepared the sample with this workaround and it is available in the below location.
Regards,
Vinothini.R