How can I add the share button to my posts?

First, copy the link of the last post in your channel and increase the number at its end by one. 

For example, if the last post is https://t.me/BotUpdates/37, then next one will be https://t.me/BotUpdates/38.

Then paste before this link the next text: https://t.me/share/url?url=

After that, add this link with URL button to your post.