<div class="container">
<select class="form_select">
<option>the option</option>
</select>
</div>
<style>
.container {
width: 300px;
border:1px solid blue;
}
select {
max-width:50px;
}
</style>
If what you're asking is how to cause a drop-down select to drop down programmatically (e.g., in response to a click somewhere else that triggers your code), you can't do that.
ref how open/close options of 'select' when you click on anyone 'div'