How to check box use iframe?

JavaScript
1

I have 2 file :
1.htm
<iframe name=“name1” id=“id1” src=“frame.htm”></iframe>

frame.htm
<input type=“checkbox” name=“checkItem” id=“checkItem_” value=“linh”>

I want to create a checkbox in 1.htm file ( write code only in 1.htm, don’t write in frame.htm)
Then I will click to checkbox in 1.htm
then the checkbox in frame.htm will checked ?
Help me!
Thanks everyone !!!