test.js

var dropDown = this.getField('MyDropDown');
var formText = this.getField('MyFormText');
formText.readonly = true;

dropDown.setAction('MouseUp', 'validate(dropDown, formText)');
dropDown.setAction('OnBlur', 'validate(dropDown, formText)');

function validate(field, textField) {
	if (field.value === 'Other') {
		textField.value = '[ Enter a value here ]';
		textField.readonly = false;
		app.alert('Please enter a value', 1, 0, 'Text Control PDF');
	}
	else {
		textField.value = '';
		textField.readonly = true;
	}
}