add checkbox checked to hyperlink

I only have an option to create 1 single link to bypass a page from a 3rd party collecting the fields within this page on the process. So far, I was able to create the link and bypass the 3rd party page by collecting all information except one - it works but it is missing this 1 field.

In this 3rd party page, there is this checkbox input unchecked where the user has the option to check it or not. I need to add to my link this value but checked, here is the 3rd party checkbox code:

<input id="flexibleDates" name="flexibleDates" class="checkbox" type="checkbox" value="true"/><input type="hidden" name="_flexibleDates" value="on"/>

I ran out of options perhaps one of you know how and it could be a silly input, but I wasn't able to make it work.

My link is, as of now, somewhat similar to where "value" is pre-set on a prior screen - again this link works for now but I really need to add the value of the checkbox: http://ift.tt/1yBMYNZ

The question is how do I add the "flexibleDates" checkbox field checked into my link?

Thanks