How to give a tag to a tnt entity fused by custom item

Right now I'm tagging every tnt entity when I fuse one tnt block with a custom_flint_and_steel. This by detecting when a custom_flint_and_steel is used and running a function:(execute as @e[type=tnt] unless score @s TNTMarked matches 1.. run scoreboard players set @s TNTMarked 2)   How do  make it so that I only tag the one I lit, while also being able to tag more with the custom_flint?