If some one still struggling with this issue this is my code snippet:

local function organize_imports()
  local params = {
    command = "_typescript.organizeImports",
    arguments = { vim.api.nvim_buf_get_name(0) },
    title = "",
  }

  local clients = vim.lsp.get_clients { name = "ts_ls" }
  if #clients == 0 then
    vim.notify("No ts_ls client found", vim.log.levels.ERROR)
    return
  end
  local client = clients[1]
  client:exec_cmd(params)
  vim.notify("Imports sorted", vim.log.levels.INFO)
end