There are a few things you could do, depending on the circumstances.
When the page is loaded, depending on where the initial focus is, you could have extra text associated with the first object (aria-describedby
) that informs the screen reader what the CTA is.
You could have a visually hidden container that is a live region (eg <div class='sr-only' aria-live='polite'>
) that is updated with text when the page loads. (Text in a live region is announced by a screen reader when the text changes.)
Whatever text you associate with the first focusable object or the live region should include the name of the object you're trying to emphasize (usually the label of a button or link) as well as the 'type' of object it is (button, link, checkbox, etc). The type is very important because as a screen reader user, I can quickly jump to objects of that type. ('B' takes me to the next button, 'U' takes me to the next unvisited link, 'X' takes me to the next checkbox, 'T' takes me to the next table, etc). I can also display a dialog with a list of objects of that type. For example, I can see all the buttons on the page or all the links on the page.
So specifying the type of the object is important. The CTA message might say "Be sure to select the Register Now button".
(I tried to keep implementation details out of my response but I needed to specify a few to make my point. If you have further questions on how to implement something like this, post on stackoverflow and make sure you add the "accessibility" tag to the question like you did with this one. That way I'll see it.)