Read request field value on the JSD request creation screen

Hi there, I want to add custom validation for the request field on the JSD request creation screen in the customer portal.
It seems like I can use this module: https://developer.atlassian.com/cloud/jira/service-desk/request-create-property-panel for validation and showing an error message. But I can’t read the value of the field on the JSD request creation screen because parent DOM is not accessible. Is there a way to read the field value?