Secure messaging (SM) is an important aspect of communication for patients with cancer. SM fosters patient-clinician communication and helps patients with symptom management and treatment support. However, patients are uncertain about how to phrase messages appropriately and have expressed the need for guidance. In response, we designed a user-centered, web-based application named SEND The application focuses on specificity, expression, needs, and directness through interactive video tutorials and quizzes. Our objective was to comprehensively evaluate SEND based on its levels of engagement, satisfaction, acceptability, and appropriateness. We recruited 101 patients with various cancer diagnoses to use SEND and then fill out a survey 1 to 2weeks later about their experience. Patients' mean age was 64years; most were male (55%), white (83%), diagnosed with cancer in 2020 with high levels of self-efficacy, and 51% had a bachelor's degree or higher. 65% were engaged in the application, and respondents spent an average of 15min interacting with SEND Satisfaction was 90.4%, 85.4% found it acceptable, and 88.6% appropriate. There were no statistically significant differences across age, sex, race, education, or year of diagnosis. Results underscore the potential of eHealth interventions, like SEND, in enhancing patient-clinician communication in cancer care. By empowering patients with effective message-writing techniques, SEND has the potential to improve the quality of SM, which can lead to faster response times and more patient-centered responses.