function sendToWebhook(event) {   
var sheet = event.source.getActiveSheet();  

if (sheet.getName() == "Sheet1") {    
var activeCell = sheet.getActiveCell();   
 
if (activeCell.getColumn() == 3 && activeCell.getRow() > 1 && activeCell.getRow() <= sheet.getMaxRows()) {      
//first value is ROW, second value is COLUMN. Negative number = upwards(row)/backwards(column)
var valueA = activeCell.offset(-0, -2).getValue();      
var valueB = activeCell.offset(-0, -1).getValue();   

//Insert your webhook URL below   
var url = "WEBHOOK_URL";      
var payload = JSON.stringify({ "valueA": valueA, "valueB": valueB }); 
         
var options = {        
"method": "post",        
"contentType": "application/json",        
"payload": payload      
};          

UrlFetchApp.fetch(url, options);      

//A notification confirming that the function has completed (lasts for 5 seconds)
SpreadsheetApp.getActive().toast("The values have been sent to the webhook.", "Notification", 5);  

}  
}
}