Hi there,
well, that is the problem with decentralized service like Mastodon. For the good or the bad, there is no zentral URL which you could use to share something.
For example, for Facebook you just have a simple link:
https://www.facebook.com/sharer/sharer.php?u=URL_TO_SHARE
For Mastodon there is no such thing, since you have to use your individuell Mastodon instance.
There are solutions out there, but all of them have the same problem: After clicking the button the user has to manually type in the URL of the Mastodon instance he or she uses. Who would do that? When you simple can Copy and Paste the URL you want so share while on a desktop computer or use the native share function of your mobile device on which you will most likely have the Mastodon app installed.
Some examples:
1. https://aly-ve.github.io/Mastodon-share-button/
2. https://codepen.io/bipoza/pen/XWbegOM
3. https://shkspr.mobi/blog/2022/06/create-a-share-to-mastodon-button-for-wordpress/
If you ever come across a Mastodon share button, where the end user does not have to enter his/her Mastodon instance URL manually, please let us know. I’ll be more than happy to include it. At the moment there is simply no useful solution I know of unfortunately.
Cheers
JP