1add_action('fluentform_before_insert_submission', function ($insertData, $data, $form) {
 2 
 3    if($form->id != 5) { // 23 is your form id. Change the 23 with your own login for ID
 4        return;
 5    }
 6 
 7    $redirectUrl = home_url(); // You can change the redirect url after successful login
 8 
 9    // if you have a field as refer_url as hidden field and value is: {http_referer} then
10    // We can use that as a redirect URL. We will redirect if it's the same domain
11    // If you want to redirect to a fixed URL then remove the next 3 lines
12    if(!empty($data['refer_url']) && strpos($data['refer_url'], site_url()) !== false) {
13        $redirectUrl = $data['refer_url'];
14    }
15 
16    if (get_current_user_id()) { // user already registered
17        wp_send_json_success([
18            'result' => [
19                'redirectUrl' => $redirectUrl,
20                'message' => 'Your are already logged in. Redirecting now...'
21            ]
22        ]);
23    }
24 
25    $email = \FluentForm\Framework\Helpers\ArrayHelper::get($data, 'email'); // your form should have email field
26    $password = \FluentForm\Framework\Helpers\ArrayHelper::get($data, 'password'); // your form should have password field
27 
28    if(!$email || !$password) {
29        wp_send_json_error([
30            'errors' => ['Please provide email and password']
31        ], 423);
32    }
33 
34    $user = get_user_by_email($email);
35    if($user && wp_check_password($password, $user->user_pass, $user->ID)) {
36        wp_clear_auth_cookie();
37        wp_set_current_user($user->ID);
38        wp_set_auth_cookie($user->ID);
39        /* user is not logged in.
40        * If you use wp_send_json_success the the submission will not be recorded
41        * If you remove the wp_send_json_success then it will record the data in fluentform
42        * in that case you should redirect the user on form submission settings
43        */
44        wp_send_json_success([
45            'result' => [
46                'redirectUrl' => $redirectUrl,
47                'message' => 'Your are logged in, Please wait while you are redirecting'
48            ]
49        ]);
50    } else {
51        // password or user don't match
52        wp_send_json_error([
53            'errors' => ['Email / password is not correct']
54        ], 423);
55    }
56}, 10, 3);