Can we add a custom Java script validation while submitting the expense report using Submit button

Summary:

We have a customer asking to add some validation in place at the submit button of the expense report, Please let us know if it is possible ? We cant see the Expenses in the Standard Objects published in the Application composer screen to add validations

Content (please ensure you mask any confidential information):

Have to create custom validation for submit button for expense reports

Version (include the version you are using, if applicable):

26A

Code Snippet (add any code snippets that support your topic, if applicable):