id of the user to send the message to (e.g., <@U1234567890>)
Write your message in markdown for nicely formatted display. Supports: headers (# ## ###), bold (text or text), italic (text or text), strikethrough (text), inline code (code), code blocks (```), links (text), block quotes (>), lists (- item, 1. item), dividers (--- or ***), context blocks (:::context with images), and section buttons (:::section-button). USER MENTIONS: To tag users, use their user ID with <@USER_ID> format (e.g., <@U1234567890>), not username.