Is there a verb form of “graffiti” in German? The closest I’ve found is “strichen,” though that’s not really what I’m looking for.
This is a contentious issue.
Those doing/making graffiti say mostly sprühen or sprayen. There are quite a few synonyms, especially within the 'scene'.
If the German target audience is that scene: orient your words on Graffiti-Jargon.
Those doing this are quite a different demographic from mainstream society. What some call (street-) art, others just call Schmierereien. The verb for that would then be schmieren, beschmieren.
The fundamental difference between those words is opinion based (and then some).
Schmieren is clearly derogatory. Sprayen not understood by many on several levels of meaning, conservatism, old people, etc.
References for "Graffiti=Schmierereien":
For a glimpse of German usage and a clouded meaning pattern:
Src: DWDS – Graffiti, das
A proper translation for to graffiti would therefore have to be sprühen. This has in my opinion the least negative or positive connotation, and is therefore also less clear-cut.
As this is about a living language, you may also ask yourself about your own opinion whether streetarten might be the ahem, more fitting translation.
'Graffiti' is an Italian loan-word in both English and German (from graffiato). It can be used in both languages and is a cognate in its noun form.
In common parlance, I'm not aware of a recognized verb form. You might consider phrasing your sentence such that an alternative verb can be used such as: "[etwas] mit Graffiti besprühen".