I want to add that, thankfully, WTBTS has not linked this verse to the field ministry. It's not an often referenced verse, either. They simply say that submitting to the good news is evidenced by generosity in giving.
At 2 Corinthians 9:13 contributions made in behalf of needy fellow Christians are shown to be an evidence of an individual’s submissiveness to the good news, it being a Christian obligation to assist needy fellow believers. - it-2 p. 1041
Yes, the good news of God’s kingdom becomes meaningful to people when they see the effect it has on the personalities of those embracing it, the generosity and love of neighbor that it generates. Such thoughtful service and giving to others not only causes these to feel gratitude toward the human givers but also “overflows in a flood of thanksgiving to God.” It recommends true Christianity as the finest way of life, as true worship of a kind and loving God. - w75 12/1 pp. 718-719