I have a homebrew amulet that I want to give my character advantage on saving throws against being poisoned and diseased. how would I do this ? or is there not a way to do this?

Modifier: Advantage->Subtype: Saving Throws->Details/Description: “against poison and disease”

I dont understand what that means. where do i do that?

You open up the item and click/tap [ADD A MODIFIER]. Then, in the dropdown labeled “Modifier” you select “Advantage,” in the Dropdown labeled “Modifier Subtype” you select “Saving Throws,” in the field leveled either “Details” or “Description” (I forget which) you type in “against poison and disease.” Then you click/tap [SAVE] on that Modifier, then you click/tap the main [SAVE CHANGES] button under Basic Information for the whole magic item.

it doesnt say that anywhere on the item when i click on it or when i hit customize

You have to do it in the homebrewer: (https://www.dndbeyond.com/homebrew/creations/create-magic-item). After you create the item you have to equip your new item to the character sheet.

Quote from IamSposta >>

You open up the item and click/tap [ADD A MODIFIER]. Then, in the dropdown labeled “Modifier” you select “Advantage,” in the Dropdown labeled “Modifier Subtype” you select “Saving Throws,” in the field leveled either “Details” or “Description” (I forget which) you type in “against poison and disease.” Then you click/tap [SAVE] on that Modifier, then you click/tap the main [SAVE CHANGES] button under Basic Information for the whole magic item.

What if my goal is to add Advantage to a saving throw without creating an extra item for that purpose?

For example, elves have advantage by default to avoid the charmed condition, and that is always mentioned in the saving throw box, underneath the saves themselves; dwarves have advantage against the poisoned condition for their saves. What I want to do is add advantage to a single saving throw type so that it's always there as a reminder whenever it's asked for.

Quote from NevadaZG >>

Quote from IamSposta >>

You open up the item and click/tap [ADD A MODIFIER]. Then, in the dropdown labeled “Modifier” you select “Advantage,” in the Dropdown labeled “Modifier Subtype” you select “Saving Throws,” in the field leveled either “Details” or “Description” (I forget which) you type in “against poison and disease.” Then you click/tap [SAVE] on that Modifier, then you click/tap the main [SAVE CHANGES] button under Basic Information for the whole magic item.

 

What if my goal is to add Advantage to a saving throw without creating an extra item for that purpose?

For example, elves have advantage by default to avoid the charmed condition, and that is always mentioned in the saving throw box, underneath the saves themselves; dwarves have advantage against the poisoned condition for their saves. What I want to do is add advantage to a single saving throw type so that it's always there as a reminder whenever it's asked for.

 
You will need to homebrew something that can add it, be it a feat, item, subclass, subspecies, etc. It is not something you can do via the ad hoc sheet customisations.

Thanks for this thread.  Was just super helpful to get a conditional advantage on saves listed in the saving throws box.