cant send a victim this link with the injected code
This might be possible if you find that some input will populate the field.
Try POST and GET requests for the page, and attempt to populate the field using either its name (if it has one), but also try
id and other variations. Have a look at the rest of the application and see if there are any commonly used parameters that could be tried here. Burp Suite Target tool is great for this because it allows you to see the site map at a glance, along with parameters sent to each page.
If you find you need to use the POST method to populate this field, then to make linking possible you would create a form on your own site to submit to the vulnerable site:
<form method="post" action="https://example.com/submit">
<input type="hidden" name="textarea1" value="<input onclick=alert(document.cookie)>" />
You could submit this automatically
and you simply send the link of your page to your victim for the reflected XSS to trigger.