I do that frequently in this forum. It works like uploading an image. Either I use the upload button or I drag it into the composer.
Though at other forums, this sometimes doesn’t work because the admins haven’t enabled the upload of video files in the authorized extensions site setting. You can tell whether only images are allowed by checking the icon in the composer: if it shows an image icon, it means only image uploads are permitted.
Here at Meta, the insert video component is installed. However, it allows further actions and is not responsible for permitting video uploads in general.
Insert Video