Dumb guide to popularity poll:
You could vote if you buy Jump #11 digital version from Jump+ or Zebrack Comic
Jump+ you need the app (or I'm too stupid to figure it out)
Zebrack Comic you can do it on the site

Part 1: Buy the magazine
Jump+
1. Make an account
jumpbookstore.com/member_regist_new.html
2. Get the app
Search 少年ジャンプ+.
*The app is region lock on the store but doesn't require vpn to use.
**Make sure you get the latest version
3. Log in
三(menu) -> ロンドン
4. Buy coins
Menu -> コイン購入
5. Buy the magazine
Tap 少年ジャンプ

Zebrack Comic
1. Make an account
zebrack-comic.com/signup/register
2. Log in
 https://zebrack-comic.com/signin
3. Buy coins
https://zebrack-comic.com/shop
4. Buy magazine
https://zebrack-comic.com/magazine/1/backnumber

Part 2: vote
Jump+
1. 少年ジャンプ
2. 詳しく見る
3. 読者アンケートに答える
4. You will see poll advertise pic. Tap that.
5. Fill the poll

Zebrack Comic
1. https://zebrack-comic.com/magazine/1/detail
2. 第二回『鬼滅の刃』人気投票はこちら！
3. Fill the form

The form
1. Character name(required)
2. Post code(required)
3. Address(required)
4. Street number(required)
5. Building name/Room number
6. Name(required)
7. Furigana(required)
8. Age(required)
9. Phone number(required)
10. Panel you want for special prize (required)
Example of writing
Tank:ＪＣ『鬼滅の刃』17巻 (vol.17)・Ｐ139 (page 139)・１コマ目(1st frame)
Magazine: 2019年(magazine year 2019)・ＷＪ15号(issue 15)・Ｐ99(page 99)・１コマ目(1st frame)

PS. If you buy the magazine before 16 Feb 11.59 pm JST., you can do the weekly poll.