OnSave.js

function OnLoad(executionContext){
  var formContext = executionContext.getFormContext();
  
  //Register custom event
  formContext.data.entity.addOnSave(validateOpportunityScore);
}

function validateOpportunityScoreOnSave(executionContext) {
    var formContext = executionContext.getFormContext();
    var opportunityScore = formContext.getAttribute("new_opportunityscore").getValue();

    if (opportunityScore < 0 || opportunityScore > 100) {
        formContext.getControl("new_opportunityscore").setNotification("Opportunity score must be between 0 and 100.");
      
      //Prevent save
      executionContext.getEventArgs().preventDefault();
    } else {
        formContext.getControl("new_opportunityscore").clearNotification();
    }
}