WordPress adding custom fields to page without plugin

Solution:1

Yes you can use the custom fields by checking from the top bar of the WordPress page admin section. However, this is not a good solution as it will likely be confusing after you have multiple custom fields which is sure.

You can use the custom meta box function for this and add the functionality to your page which is easier done through a plugin called advanced custom fields which you already seem to know.

So, if you don’t want to use the plugin instead you want to do it custom then there is a function callend add_meta_box. This function allows you to add a seperate box on a specific post_type.

For Quick Tutorial: https://www.sitepoint.com/adding-custom-meta-boxes-to-wordpress/

Hope this gives an overview on meta boxes. Thanx.

Solution:2

First of all, go to the post editor. Usually you can find regular input fields like the title, the content, the category, and the tag box there. There are a few hidden fields as well, including a Custom Fields Editor, which we can use to add our custom meta.

To display it, click the Screen Options and tick the Custom Fields option.

The Custom Field box should now appear at the bottom of the post content editor. Now, we can add our custom meta-data.

A single meta-data is handled with key name/value pairs. Hence, as you can see below, the Custom Fields box consist of two input fields: name and value.

In this following example, I will specify a new meta-data for the post that I’m currently editing, with hello_world as the key and Hello World as the value.

Click the Add Custom Field to add them in the post. You can later update or delete this meta-data. Also, as you have entered a key name before, WordPress will store and list the name in a dropdown option, so that you can reuse the key without having to type it all over again.