The young German placed sixth in the Grand Prix on Saturday night before registering the best result of his fledgling career with victory in the final qualifier of the season in the Netherlands.
Will, ranked just 134th, put on a remarkable show at the Brabanthallen to best some of the top riders in the world.
With Colorit, the 24-year-old rookie topped the podium ahead of Australian Edwina Tops-Alexander, riding Itot du Château Cevo, and his compatriot Marcus Ehning, with Copin van de Broy.
"To come here and win is really a big step forward," said Will, who won the CSI4*Munich Grand Prix at the end of last year with his 10-year-old stallion.
"Colorit arrived in my stables a bit more than a year ago, he is really competitive and constant," Will added. "He almost placed in all the grands prix we have entered."
Further down the field, Sweden's Malin Baryard-Johnsson (fourth) and Dutchman Gerco Schröder (fifth) secured their places at next month's final in Gothenburg.
But Belgium's Ludo Phillipaerts, Dutchmen Jeroen Dubbeldam and Maikel van der Vleuten, Marco Kutscher of Germany and Brit Scott Brash missed the cut.
The final in Sweden runs from April 24 to 28.
Final Showjumping World Cup final qualifying standings:
1: Sergio Alvarez Moya (SPA) 75 points
2: Christian Ahlmann (GER) 72 points
3: Kevin Staut (FRA) 72 points
4: Pius Schwizer (SWI) 70 points
5: Luciana Diniz (POR) 61 points
6: Meredith Michaels-Beerbaum (GER) 60 points
7: Marcus Ehning (GER) 59 points
8: Malin Baryard-Johnsson (SWE) 53 points
9: Denis Lynch (IRL) 53 points
10: Marc Houtzager (NED) 51 points
11: Henrik von Eckermann 50 points
12: Jens Fredricson (SWE) 48 points
13: Gerco Schröder (NED) 48 points
14: Pénélope Leprevost (FRA) 48 points
15: Roger-Yves Bost (FRA) 47 points
16: Hans-Dieter Dreher (GER) 46 points
17: Steve Guerdat (SWI) 44 points
18: Rolf-Göran Bengtsson (SWE) 43 points
Edwina Tops-Alexander (AUS) qualifies automatically as defending champion