Note: After saving, you have to bypass your browser's cache to see the changes. Google Chrome, Firefox, Microsoft Edge and Safari: Hold down the ⇧ Shift key and click the Reload toolbar button. For details and instructions about other browsers, see Wikipedia:Bypass your cache.
function resetSandbox() {
    document.editform.wpTextbox1.value = "\{\{Please leave this line alone (sandbox heading)\}\}\n" + 
"<!-- Hello! Feel free to try your formatting and editing skills below this line\. As this page is for editing experiments, this page will automatically be cleaned every 12 hours. -->" + text; //Add the base text for the sandbox
    document.editform.wpSummary.value = "[[User:Animum/reset.js|JS EDIT]]: Reset the Sandbox"; //Make a descriptive edit summary
    document.editform.wpMinoredit.checked = true; //Make it a minor edit
    document.editform.wpSave.click(); //Submit it!
}
 
function addResetTab() {
    if( wgPageName == "Wikipedia:Sandbox" && ( wgAction == "edit" || wgAction == "submit" ) ) //Add the tab for previews, edit conflicts, and bare editing of the sandbox 
      { mw.util.addPortletLink('p-cactions', 'javascript:resetSandbox()', 'reset', 'ca-reset', "Reset the Sandbox"); }
}
 
$(addResetTab);