If you are using the text editor,
just use the icons in the normal way.
If however you are presented with just a plain text box,
in order to throw a new line or insert a link or a picture, you will need to use "BB codes", otherwise known as "forum codes".
BB codes are a simplified form of "HTML codes". They make the website safer from hackers and from accidental damage by users typing HTML or simply making typing mistakes. You can google BB codes and learn them, but be aware that there aren't unified standards across the web and the codes may vary between different forums.
The codes are made up of a starting code and an ending code, in between which you enclose your text or URL or some command or other. Such code pairs are known as tags. Here are a few examples:
[p]Your text here[/p]
You can find a usefull reference on this page https://www.bbcode.org/reference.php.