br2.select_form("name_of_the_form")
br2.form['search_text'] = 'test'
br2.form['searchin'] = ['sender']
res3 = br2.submit()

<div class="search-content">
message
<input type="text" name="searchFor2" id="searchBar" value="" maxlength="40"/>
in
</div>
<table class="search-in">
<tr>
<td>
<span class="radio ib">
<input type="radio" name="searchin" value="sender" id="searchin_sender"  />
<label for="searchin_sender">sender's name</label>
</span>
</td>
<td>
<span class="radio ib">
<input type="radio" name="searchin" value="recipients" id="searchin_recipients"  />
<label for="searchin_recipients">recipient's name</label>
</span>
</td>
</tr>
</table>
</div>