。
She readily agreed with my proposal.
Literal
She [topic-は] willingly my [possessive-の] proposal [to-に] agreed.
賛成する (さんせいする, 'to agree, to approve') takes the thing agreed with marked by に — the agreement is directed at the proposal. The adverb 進んで signals she agreed without hesitation or resistance. 提案 (ていあん, 'proposal, suggestion') is a common formal-register noun used in both business and everyday deliberation contexts.