According to the rules you can't, Mark of the Sentinel arises in a Human bloodline and nowhere else.
Of course that is just the RAW answer and the allowances for NPC's to break PC restrictions is quite strong and in the end the DM has the power to do what he wants. So if you want a Warforged with a Dragonmark then go for it, read up on the lore of dragonmarks and create a compelling backstory and have yourself a great villain for your game. The only restrictive rule I'd follow here is to not let this specific quirk make for a more overpowered creature than you would otherwise have used but that goes for all NPC's you create/use so it really isn't much of an restriction at all.
My bad, I meant mechanically get it into dnd Beyond character sheet.
Mark of the Sentinel is a variant Human race in DnD Beyond's character builder. The only way to do that would be to create a subrace under Warforged with the Mark.
EDIT: Okay, a quick edit. You could also technically build a custom Feat with the same effects that the Mark would provide and manually add it.
Your are trying to do something that the rules doesn't allow for so yea, as @Athanar said, you need to homebrew a new Warforged subrace and change it to your liking.
There isn't "no way" to do this. It just isn't easy or reasonable to do it. But if you wanna get into crazy spell and magic interaction territory there is a lot of stuff you can do with high level effects that doesn't initially seem possible.
The fact that dndbeyond lacks functionality for something doesn't mean you can't do it. Dndbeyond lacks functionality for all sorts of things. Even basic raw plain options are broken in places.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
I was asked to inform you that you can’t make it as a subrace for Warforged because of technical reasons in the homebrewer. You would have to instead make it as a variant race of warforged. I don’t really know any more than that so don’t ask me why. You would have to check the faq in the homebrew forum for more info.
How can I get the Mark of the Sentinel on a Warforged NPC Villian?
According to the rules you can't, Mark of the Sentinel arises in a Human bloodline and nowhere else.
Of course that is just the RAW answer and the allowances for NPC's to break PC restrictions is quite strong and in the end the DM has the power to do what he wants. So if you want a Warforged with a Dragonmark then go for it, read up on the lore of dragonmarks and create a compelling backstory and have yourself a great villain for your game.
The only restrictive rule I'd follow here is to not let this specific quirk make for a more overpowered creature than you would otherwise have used but that goes for all NPC's you create/use so it really isn't much of an restriction at all.
My bad, I meant mechanically get it into dnd Beyond character sheet.
Mark of the Sentinel is a variant Human race in DnD Beyond's character builder. The only way to do that would be to create a subrace under Warforged with the Mark.
EDIT: Okay, a quick edit. You could also technically build a custom Feat with the same effects that the Mark would provide and manually add it.
Your are trying to do something that the rules doesn't allow for so yea, as @Athanar said, you need to homebrew a new Warforged subrace and change it to your liking.
The homebrew forum have a lot of help and people that's good at such stuff, have a look.
There isn't "no way" to do this. It just isn't easy or reasonable to do it. But if you wanna get into crazy spell and magic interaction territory there is a lot of stuff you can do with high level effects that doesn't initially seem possible.
The fact that dndbeyond lacks functionality for something doesn't mean you can't do it. Dndbeyond lacks functionality for all sorts of things. Even basic raw plain options are broken in places.
I'm probably laughing.
It is apparently so hard to program Aberrant Mind and Clockwork Soul spell-swapping into dndbeyond they had to remake the game without it rather than implement it.
I was asked to inform you that you can’t make it as a subrace for Warforged because of technical reasons in the homebrewer. You would have to instead make it as a variant race of warforged. I don’t really know any more than that so don’t ask me why. You would have to check the faq in the homebrew forum for more info.
Right. Warforged isn't designed for subraces like elves are. Variant races allow you to add or replace features of the base race.