Step1:

Switch to old Telegram web version like picture below, old version url is https://web.telegram.org/?legacy=1#/im

If you are on old version, go to step 2.

Step2:

Open extension, you will see the [Start Messaging] button on the bottom after finishing the step1.

set usernames that you want to send message. (you can get usernames by 'Scraper' feature)

Step3:

Click [Start Messaging] button to send bulk messages to those users. Stay on this window while sending!

and wait a moment, the bulk messages will be sent to them.