Digital advocacy tools allowing advocacy groups to move their activities online hold promise as a means to bolster the participation of politically marginalized demographics—groups that historically engage less in political activities, such as women compared to men (i.e. politically marginalised demographics are groups of the population that tend to participate in politics at a comparatively lower extent than others e.g. women compared to men) (Heger & Hoffmann, 2021). However, emerging evidence suggests that online advocacy actions may inadvertently perpetuate offline patterns of political engagement, amplifying the interests of powerful voices (Junk, Romeijn, & Rasmussen, 2021; Van der Graaf, Otjes, & Rasmussen, 2016). In collaboration with advocacy groups, we develop a field experiment on social media platforms to examine the impact of incorporating content that enhances public efficacy on the political engagement of marginalized communities. The experiment is a multistage randomised control trial conducted on a survey platform as well as on social media. We compare the effect of different types of content on public engagement with a petition. Moreover, we investigate individual-level variation in the effectiveness of our treatments leveraging data generated through the survey, which is subsequently linked to social media activity of the experiment’s participants.