See https://developer.yahoo.com/fantasysports/guide/ for details

transaction_propose_trade.xml

<?xml version='1.0'?>
<fantasy_content>
  <transaction>
    <type>pending_trade</type>
    <trader_team_key>248.l.55438.t.11</trader_team_key>
    <tradee_team_key>248.l.55438.t.4</tradee_team_key>
    <trade_note>Yo yo yo yo yo!!!</trade_note>
    <players>
      <player>
        <player_key>248.p.4130</player_key>
        <transaction_data>
          <type>pending_trade</type>
          <source_team_key>248.l.55438.t.11</source_team_key>
          <destination_team_key>248.l.55438.t.4</destination_team_key>
        </transaction_data>
      </player>
      <player>
        <player_key>248.p.2415</player_key>
        <transaction_data>
          <type>pending_trade</type>
          <source_team_key>248.l.55438.t.4</source_team_key>
          <destination_team_key>248.l.55438.t.11</destination_team_key>
        </transaction_data>
      </player>
    </players>
  </transaction>
</fantasy_content>