Form action URL – Plus to space

I have a form in my wordpress theme. I want add a “plus” to space in url.

My source code:

<form method="get" id="searchform" action="/search/" onsubmit="return false;">
<input type="text" name="s" id="s" placeholder="Search" />
<input type="submit" class="submit" id="searchsubmit" value="<?php esc_attr_e( '', 'themeO' ); ?>"  onclick="window.location.href=this.form.action + this.form.s.value;" />
</form>

my actual search url – keyword : “good job”

http://www.example.com/search/good job

I need this url:

http://www.example.com/search/good+job