Keep in mind no changes are made until you commit them using the 'Save' button at the top right of the map area. You also have 'Undo' available. You can also make changes to your line before saving. Right clicking the line or one of the nodes on the line will give additional options (such as being able to continue drawing your line).
Hit 'Save', add a descriptive Changeset Comment and add your source (usually aerial imagery).
If the trail is tree-covered and not visible on the map, you may have used GPS to record the track. In that case you would want to use something like the JOSM desktop editor that has more capability. It can import GPX files, etc.
[0] https://streetcomplete.app