Add item under "Members Only" in the left column of our web page?

Our club uses Square for members to make dues payments. We want to add an item in the left column under Members Only that has the link to our Square account where they can pay dues. How do we add an item to the left column named "Make Payments" and how do we add the link for out Square account? Thank you.

Please read the documentation your answer is there.

Links to other sites
support.toastmastersclubs.org/doc/item/links-to-other-sites

The website you suggested said to go to "Custom web pages." Where is that? Thank you.

in your admin console.
You really need to read all the documentation.