The last ability of Burrenton Forge-Tender doesn’t target anything. You choose a source of damage as the ability resolves, if able. You can activate it even if there is no source to choose.

If a permanent spell is chosen as the source of damage, Burrenton Forge-Tender’s prevention effect continues to apply for the rest of the turn to the permanent that spell becomes once it has entered the battlefield.

If the red source you chose changes colors before it deals damage, the damage-prevention effect doesn’t apply.