Should we pray when we witness?
2 Thess. 3:1 "Finally, brethren, pray for us, that the word of the Lord may run swiftly and be glorified, just as it is with you,"
"When such a worker offers prayer to God in the family where he is visiting, the hearts of the members are touched as they would not be by any prayer offered in a public assembly. Angels of God enter the family circle with him, and the minds of the hearers are prepared to receive the word of God; for if the messenger is humble and contrite, if he has a living connection with God, the Holy Spirit takes the word and shows it to those for whom he is laboring." Manuscript Releases, Vol. 8, 196.3
Prayer is made seeking the blessings of God upon the interchange, wisdom and the presence and power of the Holy Spirit to make the visit effective.
The following has a set of quotations on prayer and witnessing that are instructive and encouraging.