To solve this problem I add the following code to my class instance:

Code:
textField = new TextField<String>() {            
           @Override
            protected void onRender(Element target, int index) {
                super.onRender(target, index);
                // PERMITE EL INGRESO SÓLO EN MAYÚSCULAS
                if (rendered) {
                    getInputEl().applyStyles("text-transform: uppercase;");
                }
            }
        };
I also added a small example of a class that directly adds this attribute to the Textfield. I called it UpperCaseTextField:

Code:
public class UpperCaseTextField extends TextField<String> {
 
    public UpperCaseTextField(String regexText) {
        
        this.setStyleAttribute("text-transform", name);
        if (regexText != null) {
            this.setRegex(regexText);
            super.getMessages().setRegexText("Caracteres inválidos");
        }
    }


    public UpperCaseTextField() {
        this(null);
    }


    @Override
    protected void onRender(Element target, int index) {
        super.onRender(target, index);
        if(rendered){
            getInputEl().applyStyles("text-transform: uppercase;");
        }
    }
}