403提交带有特殊字符的表单时出现禁止错误

I'm trying to submit a form with the following content of textarea:

scp -P 22 ~/.ssh/id_rsa.pub remote:.ssh/authorized_keys

But it doesn't accept, the following error appears on submitting:

You don't have permission to access /test.php on this server. Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

When I try any normal text, it's accepted!

This is my code:

<form action="" method="post">
<textarea name='test' rows="10" cols="100">scp -P 22 ~/.ssh/id_rsa.pub remote:.ssh/authorized_keys
</textarea>
 <input type="submit" />
</form>

Note: I'm running on PHP Version 7.2.7 on a reseller account