The Research Software Alliance (ReSA) and the Netherlands eScience Center hosted a two-day international workshop in Amsterdam (the Netherlands) on 8-9 November 2023 to set the future agenda for national and international funders to support sustainable research software. As the importance of software in research has become increasingly apparent, so has the urgent need to sustain it. Funders can play a crucial role in this respect by ensuring structural support. Over the past few years, a variety of methods for sustaining research software have been explored, including improving and extending funding policies and instruments. During the workshop, funding organizations joined forces to explore how they can effectively contribute to making research software sustainable. In preparation of the workshop, a draft of a future international declaration, titled "Amsterdam Declaration on Funding Research Software Sustainability" was created by the Research Software Alliance, the Netherlands eScience Center and with input from stakeholders in the field.