So assuming that your custom fields are all lower case, then your code should be something like this.
import wixUsers from 'wix-users';
import wixWindow from 'wix-window';
import wixLocation from 'wix-location';
$w.onReady(function () {
$w("#submitButton").onClick( (event) => {
let email = $w("#email1").value;
let password = $w("#password").value;
let first = $w("#firstName").value;
let last = $w("#lastName").value;
let position = $w("#position").value;
let phone = $w("#phone").value;
let howTheyHeard = $w("#dropdown1").value;
let organization = $w("#organization").value;
let address = $w("#address").value;
let gradeLevel = $w("#gradeLevel").value;
let comments = $w("#moreinfo").value;
wixUsers.register(email, password, {
contactInfo: {
"firstName": firstName,
"lastName": lastName,
"email": email,
"position": position,
"phone": phone,
"how they heard": howTheyHeard,
"organization": organization,
"address": address,
"grade level": gradeLevel,
"comments": comments
}
} )
.then( (result) => {
let resultStatus = result.status;
wixWindow.lightbox.close();
wixLocation.to("/member/me");
} );
} );
});