How to trigger partial validation inside a form

### Are you submitting a **bug report** or a **feature request**?
I am not sure, but I cannot found any docs about this. the `formApi` didn't expose validation related methods.

### What is the current behavior?

My situation is one registeration form, there is a captcha is required, so when click get captcha code must trigger a phone number validation, but this validation is not cover whole form only for phone number. How could I get this?

### What is the expected behavior?
Trigger a partial validation for values inside a form

### Sandbox Link
[](https://codesandbox.io/s/9jqp00j4kp)

### What's your environment?

both lastest version

### Other information