How to add a login that accesses an outside website (Specifially OPS)

I’m currently working on a website for my employer that needs to include a login, but not to access the wordpress dashboard. Long story short, this is for a print company that uses an outside source to house their templates and individual stores for customers. I need to include a login that references this page: https://host.onlineprintsolutions.com/gandyprinters/ShowPage.asp?logingp.htm. I would like to avoid using iframes to do it. I want it to be a streamlined login in the header.

Any help is much appreciated.

Thanks
Rebecca