Create an action set it to weapon, input the ability score the save uses Eg, Strength meaning it uses the strength score to add to the save DC and then input the save the monster has to use which is the save type Eg. Constitution save the monster has to make a constitution save. Action type should be general if you don't want to add the dexterity modifier to the damage, weapon if you want to add it as a weapon whether natural or unarmed, since its dragonborn assume this must be natural, then spell for spell like effects.
How would you add a save to a race build in Beyond, like you see on a Dragonborn's breath weapon?
Create an action set it to weapon, input the ability score the save uses Eg, Strength meaning it uses the strength score to add to the save DC and then input the save the monster has to use which is the save type Eg. Constitution save the monster has to make a constitution save. Action type should be general if you don't want to add the dexterity modifier to the damage, weapon if you want to add it as a weapon whether natural or unarmed, since its dragonborn assume this must be natural, then spell for spell like effects.
For more help in creating homebrew refer to this link (A Homebrewers’ “How-To” FAQ* - Homebrew & House Rules - Dungeons & Dragons Discussion - D&D Beyond Forums - D&D Beyond (dndbeyond.com)).