The 22-year-old joined the Championship side on a 28-days loan last week and made his debut in the 2-1 win over Rotherham on Saturday.
It was Johnstone's second debut for the club, as he spent the second half of last season with the Lilywhites.
"Initially I'm only here for a month so hopefully we can win all those games," Johnstone told the Lancashire Evening Post.
"I'm hoping the loan gets extended – I want to be here until the end of the season.
"I want to play football, I'm 22-years-old. Last year, I had the first half of the season at Doncaster and the second half at Preston.
"Playing the whole season kicks you on, makes you grow up and learn more about the game.
"Coming back feels good. Nothing has changed, just a few new faces. I trained with the lads on Thursday and Friday and it feels like I haven't been away."