Solution:
Use below form:
<form method="post" action="http://yourdomain.com/blog/wp-login.php" id="loginform" name="loginform">
<p>
<label for="user_login">Username<br>
<input type="text" size="20" value="" class="input" id="user_login" name="log"></label>
</p>
<p>
<label for="user_pass">Password<br>
<input type="password" size="20" value="" class="input" id="user_pass" name="pwd"></label>
</p>
<p class="forgetmenot"><label for="rememberme"><input type="checkbox" value="forever" id="rememberme" name="rememberme"> Remember Me</label></p>
<p class="submit">
<input type="submit" value="Log In" class="button button-primary button-large" id="wp-submit" name="wp-submit">
<input type="hidden" value="http://yourdomain.com/blog/wp-admin" name="redirect_to">
<input type="hidden" value="1" name="testcookie">
</p>
</form>
Note: Replace http://yourdomain.com/blog/ with your actual blog url