Create custom field to input canonical url in a page
If you truly want to this via a custom field:

Admin &gt; Settings &gt; Fields is where you create custom fields.
Settings &gt; Sections &gt; Edit Entry Type is where you assign custom fields to your section. You drag and drop the field under &quot;Design Your Field Layout&quot;.
You can then output the custom field in your template using {{ entry.customFieldHandle }} for example.

A more common approach would be to code this tag in your templates instead of using a custom field. I can't give you a very specific approach to try with the code since I do not know your site structure and desired result.