On iPhone and iPad
On iPhone and iPad, Save to Unread and Subscribe in Unread are available in the share sheet. Both appear in the set of actions listed vertically with monochrome icons:

If you use either action frequently, you can add it to your set of Favorites as follows:
1. Tap the Edit Actions button at the very bottom of the share sheet.
2. Find Save to Unread or Subscribe in Unread in the Other Actions section.
3. Tap the green plus (“+”) sign next to the action.

Adding an action to Favorites puts the action closer to the top of the share sheet.

On Mac
After installing Unread, you will need to enable its Save to Unread and Subscribe in Unread share extensions in order to use them. You can do so as follows:
1. Open System Settings.
2. Select General in the left navigation pane.

3. Select Login Items & Extensions in the right pane.

4. Click the “i” circle in the Sharing row.

5. Turn on the Save to Unread and Subscribe in Unread toggles.

6. Click the Done button.
After doing this, you should see Unread’s Save to Unread and Subscribe in Unread share extensions in share popovers, such as this one from Safari:

Similarly you should see them in share menus, such as this one from Chrome:
