How do I spawn a block under an armor stand in Minecraft?

I am trying to make a parkour course that generates itself as you jump. However, I don't know how to make the block spawn.

I already made an armor stand follow me with this command:

/tp @e[type=minecraft:armor_stand] Sieniz 
                                  (my name)

but how do I make the block spawn under me? I know how I could do it but I don't know how should the commands go. Can somebody help?

I am trying to do it in version 1.16.4.

Solution 1:

 /execute at @e[type=minecraft:armor_stand, name=joe] run setblock ~ ~-1 ~ minecraft:dark_prismarine

You can skip the entire armorstand shenanigans and address @p (the player) instead.