<form id="register_form" enctype="text/plain" name="form_prev" method="GET" action="http://some/url/path" target="_blank" class="ng-pristine ng-valid">
...
<input name="signature" type="hidden" value="YT4M_тут_много_цифр _и_букв_0NGI=" id="signature">
</form>
<button class="btn btn_default btn_default_block" id="submit-register"
onclick="document.getElementById('register_form').submit();">Далее</button>
http://some/url/path?sector=...&signature=YT4M_тут_много_цифр _и_букв_0NGI%253D
<form id="register_form" ... /form>
<button ... onclick="document.getElementById('register_form').submit();">Далее</button>