First, make sure to save the agenda template you want to share.
1. Click on “Tools” then "Template Library” in the left menu.
2. Click on the tab "Publish template".
3. Select "Publish" for the current template in the list.
4. Write title, description and set creator.
For privacy level, you can set "Private" (then the template can be downloaded by all users in all boards you are part of) or "Hidden" (then the template can be downloaded by only you in all boards you are part of). Then press "Publish".
Ignore the next step where the template can be shared with other users.
5. Switch to the other board where you want to download the template.
6. Click "Tools" then "Template Library" in the left menu. Go to "Import Template" > "Private/Hidden Templates". Click "Save" for the current template.
Now the template is available to you (and other users on the board) when you need to create a new agenda.