app.component.ts

/**
 * makes the field required if the predicate function returns true
 */
function requiredIfValidator(predicate) {
  return (formControl => {
    if (!formControl.parent) {
      return null;
    }
    if (predicate()) {
      return Validators.required(formControl); 
    }
    return null;
  })
}

//in ngOnInit: 
this.myForm = this.fb.group({
  myCheckbox: [''],
  myEmailField: ['', [
     Validators.maxLength(250),
     Validators.minLength(5),
     Validators.pattern(/.+@.+\..+/),
     // <--- here we pass predicate arrow function
     requiredIfValidator(() => this.myForm.get('myCheckbox').value) 
  ]]
})