Insert user.id to database (code)

try this

import wixUsers from 'wix-users';
import wixData from 'wix-data'; 

let user = wixUsers.currentUser
let userID = user.id

$w.onReady(function () {
});

export function button2_click(event){
 let tosave = {'fieldname': userID}
    wixData.insert("dataset",tosave)
}