Please note that bare URL footnotes – i.e. footnotes that contain only the website http address – are susceptible to link-rot, which means that if the website moves to a new domain with a new URL, the link will be broken and readers will be unable to read the citation.
First put the cursor at the point in the edit text where you want the footnote to go, then click "Cite" in the edit strip at the top of the Edit Page, then click "Templates" on the left, and a drop-down menu appears.
Choose "cite web" or "cite news" (for articles and websites), "cite book" or "cite journal", click and a box comes up.
Fill in the all details of the citation, then click "Preview" and "Show parsed preview" to see it looks right. (To correct anything, correct the box entries, then click the two "Previews" again.) In "cite book" remember to add the page number(s) of the book.
Click "Insert" and the citation automatically goes into the edit text. (It may not go in at the exact point where the cursor is if you use Firefox or Chrome.)