I found the same as the OP finding but this is a different platform name, the OP posted a fake sales serum and now I find Westarter fake sales the trick is the same, nothing is different.
They collect your Telegram usernames from different groups that you join. Then they give you a message by attaching such fake website. So you too have received the message of a similar type scam.
Not sure how legit the Serum project is but here is the official link to their website: https://projectserum.com/
Serum is a popular and legit project. Because of the popularity of this project, scammers are using the name of this project to scam. When a project is popular in the crypto market, scammers use the name of that project very simply to fool people. I was in real Serum's official telegram group so the scammer got my telegram account from that official group.