A minor note: you can access D&D Beyond content without owning it if you're in a campaign on D&D Beyond with someone that owns that content, has a Master Tier subscription, and has content sharing enabled for that campaign.
That said, I assume that's not what you're asking.
For individual items, monsters, and spells, just find them in the respective listing
Magic items, mundane equipment, spells, and monsters each have their own individual listings on D&D Beyond. You can view these overall listings even without owning the specific content that appears in those listings - including the name, source, and some basic information shown in the corresponding columns.
Only viewing the full details of the item/spell/monster requires owning the corresponding content (or having it shared with you), and only for non-SRD content that hasn't been released for free (content from the EEPC or the Haunted One background, for instance, can be freely accessed).
Even without being able to access the full description, though, if you know the thing you want to link to is relevant, you can simply right-click on the item in the list to copy the URL of the linked entry.
For anything else (or the above), use the D&D Beyond search
You can use the D&D Beyond search function to find basically anything else that's on D&D Beyond. The search results will show you a small preview of the relevant text, and each result will point to one of the above listings or to the relevant section heading for D&D Beyond Compendium content. By default, it searches all D&D Beyond content, but you can filter it to just character options, compendium content, spells, items, monsters, forum results, or any combination thereof.
You can see these previews even without owning the corresponding content, but again, you won't be able to see the full thing without owning/having access to it. You can follow the same process as above to get the links; simply right click on the header by the relevant result, and copy the URL.
You can follow the above processes to find the relevant links regardless of whether you own the corresponding content, though you'll need to own it or have access to it to actually view those full details - so if you can't actually access it on D&D Beyond, you should know what you're looking for beforehand (e.g. from the physical book) so that you know you're getting the right link.
You can even follow this process regardless of whether you're logged in to D&D Beyond. All of the above (minus viewing full details of non-free content) can be done while logged out, as I've just tested in an incognito browser window.
PS: Make sure to include page numbers for (non-free) content where you can, in addition to D&D Beyond links. That way, those who own the source can verify the information, no matter how they own it. See this conversation on meta for more information.