Adding a button to header on the Bedford template for version 7.0

Hi!  I have absolutely no experience coding, but I unfortunately need to code to be able to embed a pop-up donation on our nonprofit's website.  The fundraising platform we just adopted has very clear instructions for how to set it up, but it involves adding a URL parameter to a button in a header, and I've just learned that the template we have (Bedford for version 7.0) does not have a button.  It only has a page as the last part of the navigation, and the URL parameter does not work with that.

I've been advised by Squarespace that I need to use custom code to add a button to the header.  Squarespace tells me this is a pretty basic thing, but not having any coding experience or training, it's super intimidating.  Can anyone help (with instructions meant for somebody who knows nothing about coding)? 

You mean click on button >> open a new page, or open a popup?

If new page, just add a Link item to Navigation then share site url, we can give some lines of code to turn link to button

If popup, do you have an popup donation code? We can test it easier

Sorry I didn't see your response until now!  You were right; I just needed to add a link item to the navigation.  The donation form was a pop-up, but the fundraising platform provided the footer code, which I assume is what made it work.  Thank you so much for taking the time to respond so thoughtfully!

Okay. Let me know if you still have any other problems in the future