Where does “tarjo-” come from?

tarjo- (Finnish) comes from Finnish -ntä, from Proto-Finnic -nta — Forms action nouns from verbs.

Ancestry of “tarjo-”, step by step

Step	Language	Word	Meaning
1	Finnish	-ntä	Front vowel variant of -nta; API
2	Proto-Finnic	-nta	Forms action nouns from verbs