This is the 9431st most frequent Swedish word.
frivilligt
Voluntarily.
Here, 'frivilligt' is used as an adverb modifying the verb 'deltog' to indicate the manner of participation.
Jag deltog frivilligt i projektet.
I participated voluntarily in the project.
In this sentence, 'frivilligt' acts as an adjective describing the state of the action 'svara' to mean optional or discretionary.
Det är frivilligt att svara på enkäten.
It is optional to answer the survey.
'Frivillig' in this phrase functions as part of the expression meaning 'voluntary basis,' describing the nature of the support being offered.