Modify the labels of the Opportunity Close Quick Create form dynamically according to the context of the opportunity closure.

OpportunityCloseQuickCreateForm.js

function onLoad(executionContext){
    var formContext = executionContext.getFormContext();
    var pageContext = Xrm.Utility.getPageContext();
    var isWon = pageContext.input.data.param_won;
    if(isWon) formContext.getControl('actualend').setLabel('Win date');
    else formContext.getControl('actualend').setLabel('Lose date');
}