How to set text a select html with jquery?

It's impractical to replace the whole content between <select id="country"> and </select> with 'Please select in here' ? If you just want to replace the first option's content, then try this.

$(window).load(function () {  
  //$("#country").html('Please select in here');
});

$(document).ready(function() {
  //$("#country").html('Please select in here');
  $("#country").find("option:first-child").html('Please select in here');
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<select id="country">
<option value="None">-- Select --</option>
<option value="China">China</option>
<option value="United State">United State</option>
<option value="Malaysia">Malaysia</option>
</select>

Please select in here:
<select id="country">
<option value="None">-- Select --</option>
<option value="China">China</option>
<option value="United State">United State</option>
<option value="Malaysia">Malaysia</option>
</select>

You can't add text in the select. You need to add in front of it.

If you really want to add the text after load, put one more div in front of the select to control

I am not entirely sure what you are trying to do, but if you just want to change the default text, you probably are looking for something like this:

<select id="country">
    <option value="None" selected>Please select in here</option> <!-- set the default selection -->
    <option value="China">China</option>
    <option value="United State">United State</option>
    <option value="Malaysia">Malaysia</option>
</select>

and then the JS to set a new value would be:

$(window).load(function () {
    $("#country").val('China');
});

Bit confuse what you trying to achieve

To Pre append

$('#country').prepend('<option>Please select in here</option>');

To dynamically Select:

$('#country').val("None")

Add new Option:

$('#country').append(new Option('Foo', true));