Yes, I added a function in the onclick event in "submitbtn" button, but not sure if this is the standard way or if there is a better way to do this. Are you using forms in Umbraco 9? If that is the case, I made this:

//* Adds a listener to onclick event in order to be able to validate
//* data and prevent to submit the form in case data is missing.
const addSubmitOnClick = () => {

    var formName = document.getElementsByName("FormName")[0].value;
    if (formName == "Opportunity Page")
        document.getElementsByName("submitbtn")[0].onclick = validateOpportunityRegistration;}

I have a field type that uses a js, then added that field type to the form and the code above to that js. You can return false in the function used to validate data (validateOpportunityRegistration in my case).

Note: addSubmitOnClick(); is called when the js script is loaded.