Hi all! I gave my druid player a magic item, that requires attunement and grants them ability to cast the Summon Beast spell without a material component. Also while attuned to it, the spell is always prepared. 

Can I create such item in dndbeyond?

Do you intend for that spell to require the caster to expend a spell slot? If so, then “yes,” but it won’t look pretty and the player will have to manually check off the spell slot.

Yes, they will need to use spell slot. But how exactly can I make this homebrew so the spell will be displayed as always prepared in their spell list? Like oath spells in paladins list, for example

You would have to add it directly to a homebrewed version of their subclass. What class are they?

If you add it through a magic item it will display as “at will,” and as I said the player will have to manually check off the appropriate spell slot.