Thanks for reaching out aazing,
If you're entering text on a website, the website may have its own controls for formatting. Command + I, will work to italicize text in a number of word processing and text editing applications.
To learn more about keyboard shortcuts in Safari, check out the following page: Keyboard shortcuts and gestures in Safari on Mac
For guidance making edits in a text-entry field on a website, we recommend contacting the developer of the website for guidance on the options available on their site.
Alternatively, you can use a word processor app like Pages, or a text editing app like Text Edit, to write and format text to your preference, than copy it into the website where you wish to post it.
Copy
•Press Command-C.
•Or choose Edit > Copy from the menu bar.
•Or Control-click (or right-click) the item you selected, then choose Copy from the shortcut menu.
Cut

•Press Command-X.
•Or choose Edit > Cut from the menu bar.
•Or Control-click (or right-click) the item you selected, then choose Cut from the shortcut menu.
You can find these steps outlined, here: How to copy and paste on Mac
We hope this helps!