Ah, I see. Yes, the share button will always take the current URL is none is provided. There’s not a way to disable the url parameter in this case.
An alternative would be to use the Hashtag button variation:
Or, you could use your own link to the Intent directly, configuring the URL parameters as you please.
Keep in mind that the text you’re setting up is going to require the user to edit it before they can share, I’m not sure how much you’re going to gain with the tweaks. The text part will be auto-selected in the browser, with the URL and Hashtag not, so that the user will type over the placeholder text but not the URL itself. With your alternate formatting you increase the amount of effort someone has to take to actually share the link, which we know from experience will result in less shares.
Ben