im having trouble updating a feature layer of mine using react, in the website documentation: Documentation
it says to do some simple things, a simple post request with attributes attached, yet nothing worked, when im trying to send a post request all i get is just a preview of the page, like the request is not post, maybe im doing something wrong, here is the code:

const updateFeature = async (objectId: number, newMuniValue: string): Promise<void> => {
    const layerUrl = 'feature layer url goes here';
   
       const updateParams = {
           f: 'json',
           features: JSON.stringify([{ 
               attributes: { 
                   OBJECTID: objectId, 
                   Muni: newMuniValue 
               } 
           }]),
           token: token
       };
   
       const requestOptions = {
           method: 'POST',
           headers: { 'Content-Type': 'application/x-www-form-urlencoded' },
           body: new URLSearchParams(updateParams)
       };
   
       try {
           const response = await fetch(layerUrl, requestOptions);
           const data = await response.json();
           console.log('Update response:', data);
           if (data.updateResults && data.updateResults[0].success) {
               alert('Update successful');
           } else {
               alert('Update failed');
           }
       } catch (error) {
           console.error('Error updating feature:', error);
           alert('Error updating feature');
       }
   };

then im using it in a button, the token is valid

can you help with construct the post request to be valid and working? perhaps they changed something in their way of updating a rest api ArcGIS feature layer and im not aware.