I'm making a race that has a breath weapon(Like dragonborns) and I want the save DC to be  8+CON+Proficiency, but I can't figure out how to add that in the builder, any tips?

The easiest way would be to create your species based on the Dragonborn (selecting that when you choose to create the species). Note that those are "Options" in the "Breath Weapons" trait.

Essentially, there are two parts to this:

1. You'll want to add an Action for the breath weapon and select "General" from the dropdown.
   1. Select the ability score you want to use to calculate the save from "Ability Score Type" (e.g., CON)
   2. Check the "Is Proficient" checkbox to let it know to use your Proficiency Bonus in the calculation.
   3. Select the ability type you want to make the enemy roll (e.g., "DEX" for the typical breath weapon) for "Save Type"
2. In the Snippet, you can use {{savedc:con}} to automatically calculate the saving throw DC based on Constitution. This will show the correct number on your Character Sheet.

The pinned posts in this forum provide some great How-To Guides that explain how some of these things work and there's another one for Snippet Code.

I thought that's how it worked but when I did it, it just said the DC was 8, not matter how high the characters CON was

If you made the change after creating it, things don't always update quite right. Make sure to save the action, the feature, and then save the overall species. You should also remove it from the character sheet, refresh the page, and then re-select the species. You should also close, reopen, and refresh the character sheet.

If that's still not working, can you show take a screenshot to show me what the action looks like? I might be able to provide more help from that.