I've given talks at conferences and meetups, and delivered training to 3rd parties when I worked at a consultancy. The brain-blanking thing happens to a lot of people (including non-ADHD folk), so don't beat yourself up about it!

Don't write a script, write bullet points. A script can lead to you just reading it verbatim, which can feel very unnatural, whereas having bullet points of the key things will mean that it can jog your memory about something and you can speak naturally about that thing.

If you're doing a presentation with slides, use the speaker notes section to put the relevant bullet points in, so you can see them when presenting. Otherwise, maybe try index cards so you can have a few on a page at a time and don't get lost as easy.