FF-dynamicDropdownField.php

// Add dynamic data to fluent form
add_filter('fluenform_rendering_field_data_select', function ($data, $form) {
    // Add data to formID 91, you can change the id with your particular form id
    if ($form->id != 91) {
        return $data;
    }
    // select only the dropdown name attribute
    // change 'dynamic_dropdown' name with your fields dropdown name
    if (\FluentForm\Framework\Helpers\ArrayHelper::get($data, 'attributes.name') != 'dynamic_dropdown') {
        return $data;
    }

    $data['settings']['advanced_options'] = array_merge($data['settings']['advanced_options'], [
        [
            "label"      => "Dynamic Option 1", // This is field label
            "value"      => "Dynamic Option 1", // This is field value
            "calc_value" => ""
        ],
        [
            "label"      => "Dynamic Option 2", // This is field label
            "value"      => "Dynamic Option 2", // This is field value
            "calc_value" => ""
        ]
        // You can add as many data you want, just add with  a comma, just like above

    ]);
    return $data;
}, 10, 2);