Mlem dev here. We aren't associated with the Lemmy developers :) We set up !mlemapp@lemmy.ml shortly after the Reddit API Exodus, and chose to host our community on .ml because it advertised itself as a FOSS-centered instance. We've considered moving to a self-hosted instance but haven't got around to it yet.
Because you can see the same comment from many different Lemmy websites, there are many different links that point to the same comment. The website only accepts links from the same instance as the comment author.
On desktop, you can get this link by clicking this button:
Bu default, links copied within Mlem will be from your instance, not the author's instance. You can customize this behavior by going to the Settings tab -> Media & Links -> Share Links.
Selecting "original instance" will cause all links you copy to be the ones that Lemvotes requires.
If it's the bug where the submit button is greyed out, you can work around it by adding a space to the title and then removing it again. That issue is fixed in the beta :)
I'm one of the Mlem developers; I'm glad you're enjoying it :) You could also consider trying the Mlem beta if you haven't already. It's about 8 months' worth of development ahead of the App Store version. The reason why we haven't updated the App Store version in so long is because the beta version doesn't have all of the moderation tools yet (due to it being a full rewrite), and we don't want to take those tools away from moderators who use Mlem.
Mlem dev here. You can attach a link to the post directly by clicking the link button in the post editor toolbar then tapping "Paste" (with a link copied to clipboard). We're aware that this isn't the most obvious UI, and are improving the design in the next version.
PieFed can do this