Robots are increasingly being used in a wide range of sectors, for example industry, care, and education. To further support this development, flexible and accessible interfaces appear to be a promising avenue allowing users without extensive programming experience to design robot-based applications. While research prototypes of such interfaces already exist, they are limited to specific use cases or application domains. However, approaches to robot programming for end-users that are applicable across different domains would bring significant benefits as robots in different application domains often perform similar tasks. Addressing this gap in existing research, we present a prototype of the ROBxTASK platform for cross-domain robot programming by end-users. The centerpiece of the platform is a visual programming environment (VPE) based on Google Blockly that consists of an extensible set of blocks allowing end-users to define and exchange complex sequences involving robot-based tasks for different application domains. The development of the platform is embedded into an extensive human-centered design process including initial stakeholder workshops and a series of user studies. Upcoming evaluations of revised versions of the platform and its VPE will involve the implementation of use cases by end-users in real-world scenarios.