How do I center a text box

By customising your theme’s CSS. Try using Firefox with the Firebug add-on for this kind of CSS work.
https://getfirebug.com/

I’m only using the box on one page.

Surely there must be a bit of html I can insert on the page to move that box from being justified left to justified center?

Yes. it’s called CSS.
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With all due respect esmi, I’m looking for someone who who knows what I need to know rather than a web directory.

Esmi provided you what you needed. This is as self-help forum after all.

Surely there must be a bit of html I can insert on the page to move that box from being justified left to justified center?

It’s the CSS that needs adjusting. Use Firebug to identify what CSS rules need to be updated first. Once you’ve got that adjust the settings in your style sheet.

Actually you are both wrong. It was simply a case of inserting align:center;margin: into the box’s html. I figured it out myself. here’s the box – https://myperfectpitch.com/text-box-page/

There you go, you’ve learned something.

Actually you are both wrong. It was simply a case of inserting align:center;margin: into the box’s html

I’m glad you were able to fix your CSS problem yourself, good job! ??