Hi abduljabbar,
Some advices:
Your submit button should be a Submit Form action and your screen should be configured as a Form with Form Action Type = email.
Then in the _custom.js a phoneui.preSubmitForm_m1_<app>() and phoneui.postSubmitForm_m1_<app>() functions are created.
The phoneui.preSubmitForm_m1_<app>() function is where validation is to be performed. Return false to discontinue the form submit process, you just need to integrate a simple validation with javascript.
Hope this make sense.