hs_hiddien_fields.js

var mapping = { 
  'last_lp_brand' : 'SEAT' 
} ; 


window.addEventListener('message', event => {
 if(event.data.type === 'hsFormCallback' && event.data.eventName === 'onFormReady') {
    console.log(">> Form is ready!");
    Object.keys( mapping).forEach(function(key) { 
      var input = 'input[name=' + key + ']'; 
      if ( jQuery( input ).length === 0 ) { 
        console.warning( ' field ' + input + ' was not found !!' ); 
      }
      jQuery( input ).first().val( mapping[key] ); 
      console.log( '>> form field ' + key + ' set to value ' + mapping[key] ); 
    }); 
 }
});