button adds page name to url

When I add an URL to a button, the button itself adds my site name to the beginning of the URL which causes an error. On the button menu I insert desired URL in the box, but instead of linking to https://desiredURL, the button links to http:/mysitename/mypagename/desiredURL. How do I prevent the button from automatically adding this to the beginning of each URL?

My site is a sales affiliate for another site, and I have links to various products on the company’s site. I am trying to use buttons to make this look better, but it keeps trying to link to something on my site which is not correct.

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