Now when they had gone through Phrygia and the region of Galatia, they were forbidden by the Holy Spirit to preach the word in Asia. 7 After they had come to Mysia, they tried to go into Bithynia, but the Spirit did not permit them. 8 So passing by Mysia, they came down to Troas. 9 And a vision appeared to Paul in the night. A man of Macedonia stood and pleaded with him, saying, “Come over to Macedonia and help us.” 10 Now after he had seen the vision, immediately we sought to go to Macedonia, concluding that the Lord had called us to preach the gospel to them. — Acts 16:6-10.
God prevents the missionaries going to Asia or Bithynia and in Troas appears to Paul during the night in a vision of a man from Macedonia asking for help. This is taken as God’s call to His servants to preach the gospel to the Macedonians.