A limitation of the text box for the hyperlink field is that it accepts only 256 characters. (Insert, Hyperlink dialog)

You can hyperlink long URLs (or edit short URLs without opening the Insert Hyperlink dialog) by pasting the URL into the message body, pressing space to convert it to hyperlink and editing the display text.

My method to edit the text: click in front of the last character in the link and use Shift+Home to select the beginning of the URL. Type the desired display text then select and delete the last letter. If your newly typed text is not hyperlinked, use Undo and leave the first and last characters until after you enter your text

In many cases the long URL contains a lot of extra data you do not need – these are often things like &source= or &rls= etc. The & and everything to the right of it can usually be removed without affecting the usability of the link but most people don’t – they send what appears in the address bar.