You can post a clickable hyperlink two different ways:
On the desktop site, click the hyperlink icon (looks like a linked chain with a green + icon), paste the URL into the "URL" field, insert what you want the link to say in the "display text" field (optional) and then click "Insert"
On desktop or mobile, simply paste the URL into the text field. The URL will be displayed as a clickable hyperlink once you click or tap "Post"