Boothroyd is hoping to end a year-long wait by bringing the Dutch forward to Vicarage Road on loan until the end of the season.
The John deal should be sealed regardless of whether Marlon King's proposed £5million sale to the Cottagers goes through or not.
Boothroyd said: "I tried to get him a year ago last January and he turned us down.
"When somebody does that, it's like a woman saying no to you. It just makes you even more determined to get them.
"It's been agreed whatever happens with Marlon. He's a massive talent with great physique, he's exceptionally quick with a great goalscoring record.
"Hopefully he'll come here and the environment that we've got will help him thrive."