If you would like your users to be able to login directly from your reseller website rather than going to the transmitsms.com sub domain you can use this very simple form code.
<form method="post" action="https://SUBDOMAIN.transmitsms.com/login">
<input type="email" name="email" required>
<input type="password" name="password" required>
<input type="submit" value="Login" name="loginSubmit">
<a href="https://SUBDOMAIN.transmitsms.com/password-reset">Reset password</a>
You will need to replace SUBDOMAIN with your reseller sub domain name. You will need to add your own CSS but this is the basic form code required.