Click Forms and select Text Field.
At Input Tag Accessibility Attributes click OK and at Add form tag? click Yes.
You have created a basic form.
Now click on a Submit button.
Click Add behavior icon...
If you dont' see it press Shift + F4.
Repeat the procedure for all other forms.
In my case there is only one left.
Save the document (Ctrl + S) and press F12 to see the page.
Don't enter anything just click on the button.
Find The following error(s) occured: code and change it.
I wrote "Please enter Username and e-mail address".
Save the document (Ctrl + S) and press F12.
You can see that the text is changed.