GeckoWebBrowser如何标记Captcha复选框（C #Windows表单）？

I'm working on a Windows application with GeckoWebBrowser and I'm trying to check the checkbox of a Captcha via code. Programmaticaly I already can get and set html elements, but this checkbox I can't reach. I can't find it in any place in the page. I'm not trying to decifrate or solve the captcha, just check the checkbox element and after that verify if it is checked. Simple like that.I'm working on a Windows application with Gecko