* Posted net loss of $475.4 mln in 2018 on $125.3 mln revenue. : HONG KONG/BEIJING (Reuters) - Luckin Coffee Inc, the Chinese challenger to Starbucks Corp, filed on Monday for a U.S. initial public offering (IPO) through which, sources said, it is looking to raise up to $800 million. The Beijing-based coffee chain set a placeholder amount of $100 million to indicate the size of the IPO and did not disclose the number of shares it would offer, a filing bit.ly/2UtnC0g with the U.S. Securities and Exchange Commission showed.