The two launched a new campaign on Monday when they put their signatures on a sports jersey to highlight the latest in William's brainchild, United for Wildlife.
Asking #WhoseSideAreYouOn?, William said, "Our plea is this: Join us and help stop illegal poaching."
Using the power of sports and some of its stars – such as Beckham, tennis player Andy Murray and motor racing driver Lewis Hamilton – as ambassadors of the charity, William hopes to inspire a new generation of wildlife supporters.
Beckham is the lead ambassador, with William's brother Prince Harry also serving as an ambassador.
In September, William announced the creation of the global alliance of seven influential conservation organizations, followed in February with a set of ideas to improve protection and reduce demand for ivory and other products.
The focus now is on publicity. "Illegal wildlife trade thrives because it is hidden. We wanted to find a way to show the world what was happening," William said, urging others to "play their part" in sporting events that might help the cause and to use social media to spread the word.
"Our generation and the younger generation can really, really make a difference and we need to do it now," said Beckham.
At the gathering, Emmanuel de Merode, director of Virunga National Park in the Democratic Republic of Congo, said it is not just animals who have been targeted by poachers – 130 park rangers have been killed in the last decade. In April, he was shot twice.
Charlie Mayhew, chief executive of the Tusk Trust, of which William is patron, tells PEOPLE that while the poaching is "relentless," headway is being made with governments, especially in the areas where consumers purchase illegally trafficked wildlife products.
William's involvement is crucial, Mayhew stressed. "This is an example of how William can use his position to draw in the support of all these amazing names. He recognizes that he is in a unique position to do so."