sealing wall/plank joint in outside stairway landing

I need weatherproofing/sealing advice.
I am replacing a plank in an exterior stariway landing. It will abut the outside stucco wall of the house (Los Angeles). The old planks have some kind of pretty heavy duty stuff that keeps water out where the plank meets the wall. There are photos of it here: /
What product should I use and how is it applied, to properly seal up the new plank at the wall?
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