I Need Help For CODE - Autoplay - Required Fields please help me

Sorry if I am writing in wrong category
Please Help me Somebody I need so much this help
I use Autoplaymedia studio to create app...
but I don't know how to code this thing


for example is SUBMIT 'button'
and
white area
like this



so when somebody don't write the email and click submit... automatically to appears this messgage
Please fill out this field.


Can help me somebody please .... ?