You mean like a share button on a site that opens the lemmy Create Post page, on a user's registered instance, with link address and title pre-filled?
I think the federated nature of Lemmy alone makes this more trouble than it's worth, and you'd need to consider that Lemmy users don't all use the same web interface - some use Photon, some use Mlmym, some use Alexandrite etc. These changes would need to be introduced in each individual one most likely...
I think it's easier just to allow users to share the link to lemmy themselves. On desktop they can copy the link, on mobile they can share to Jerboa or whatever app, just provide the buttons for either I guess