Create or open a Google Sheet. In the menu: Extensions > Apps Script.
Paste the code below into the script editor and click Save.
Deploy: click Deploy > New deployment, set type to Web app, execute as Me, who has access Anyone. Copy the Web app URL.
Paste the URL above, click Test to verify, then click Save & Enable.
function doPost(e){
var ss=SpreadsheetApp.getActiveSpreadsheet();
var sheet=ss.getSheetByName('Leads')||ss.insertSheet('Leads');
var data=JSON.parse(e.postData.contents);
var headers=['created_at','first_name','last_name','email','phone','age','goal','beneficiary_name','zip','state','utm_source','utm_medium','utm_campaign'];
if(sheet.getLastRow()===0) sheet.appendRow(headers);
var rows=Array.isArray(data.leads)?data.leads:[data];
rows.forEach(function(l){ sheet.appendRow(headers.map(function(h){return l[h]==null?'':l[h];})); });
return ContentService.createTextOutput(JSON.stringify({ok:true,added:rows.length})).setMimeType(ContentService.MimeType.JSON);
}