If you want to display a login box on your website from where your Customers and Sub-Resellers can login into their respective Control Panels, then you may integrate the following form on your website:
<input type="hidden" name="redirectpage" value="null">
<input type="hidden" name="currenturl" value="https://<Your Control Panel Branded URL>">
<input type="hidden" name="pid" value="
<Your Reseller Id>">
<input type="text" name="username" value="" size="30">
<input type="password" name="password" size="30">
<input type="hidden" name="role" value="<role>">
<input type="submit" value="Login" class="submit">
<Your Control Panel Branded URL>: Here you need to mention your Control Panel Branded URL. You can check the same from within your Reseller Control Pane. Point to Settings ->Branding Settings and then click Control Panel. You can either use the Partially Branded URL or the Fully Branded URL. See details
<Your Reseller Id>: You can get your Reseller Id from within your Reseller Control Panel by going to Settings -> Personal Information and then click Primary Profile. Here, the first field is the Reseller Id. You need to mention this number in place of <Your Reseller Id>.
<role>: If you are integrating Sub-Reseller Login, then you need to put in the role as reseller and if you want to integrate Customer Login box, then put the role as customer.