Background: Informal caregivers of persons with chronic conditions such as heart failure have a crucial role in providing support. They often providethis help and care without formal training orfinancial compensation. Theirsituation can be experienced as demanding and complex, and caregivers have expressed a need for support. In response to their needs, an online support program was co-designed with and for caregivers. The co-design process ensured that the content is relevant to their needs and preferences. However, relevance alone does not guarantee that the program will be feasible or perceived as helpful in caregivers' everyday lives.

Objective: This study aimed to explore the experiences of informal caregivers of persons with heart failure who had access to an online support program and to identify facilitators and barriers in order to reflect on implementation strategies for scaling up use.

Methods: Participants in this study were included from a multicenter randomized controlled trial in which the co-designed support program is being evaluated. This study used an abductive approach to explore caregivers' experiences and to identify barriers, facilitators, and implementation strategies. Fifteen informal caregivers participating in the randomized controlled trial were interviewed for this purpose. The interviews were analyzed using qualitative content analysis, and the findings were subsequently mapped onto the Digital Health Engagement Model (DIEGO).

Results: This study identified caregivers' experiences of engaging with the support program, as well as factors influencing their use of the online content. Drawing on these experiences and applying DIEGO, reflections on tailored implementation strategies were formulated. These strategies emphasizethe importance of active outreach to raise awareness of the support program, securing endorsements from relevant sources, and the continuous adaptation of the intervention to enhance its feasibility and relevance in caregivers' everyday lives.

Conclusions: A modified version of DIEGO proved valuable for identifying implementation strategies based on caregivers' experiences, with the aim of scaling up use of the support program. Furthermore, DIEGO supported the identification of factors that may provide valuable insight into the implementation process, both in clinical practice and in research.