<form method="get" action="/main"></form>

Select your city:
        <select name="cityFilter" ...>
            <#list cities as city>
                <option>${city.cityName}</option>
            </#list>
        </select>

<select id='select_id' onchange='invokeJsCode()'>...</select>

<script>
function invokeJsCode() {
   $.post('/path_to_controller', {
     selected: $("#select_id").val()
   })
}
</script>

<script>
        function invokeJsCode() {
            $.get("/branches", {
                selected: $("#select_id").val()
            })
        }
    </script>

<select id="select_id" onchange="invokeJsCode()">
    <option>Moscow</option>
    <option>London</option>
    <option>Paris</option>
</select>

@GetMapping("/branches")
    public String branchePage(@RequestParam(name = "select_id", defaultValue = "All") String cityFilter,
                              @AuthenticationPrincipal User user,
                              Model model) {
        model.addAttribute("user", user);

        List<City> cityList = cityRepo.findAll();
        model.addAttribute("cities", cityList);

        if (cityFilter.equals("All")) {
            List<Branch> branchList = branchRepo.findAll();
            model.addAttribute("branches", branchList);
        } else {
            City city = cityRepo.findByCityName(cityFilter);
            List<Branch> branchList = branchRepo.findByCity(city);
            model.addAttribute("branches", branchList);
        }

        return "branche";
    }