Use Scriptrunner for JIRA Cloud to add the project's lead as a watcher

watcher.java

def issueKey = issue.key
def projectKey = issue.fields.project.key
def projectLead = get("/rest/api/2/project/${projectKey}").asObject(Map).body['lead']['key']

logger.info("Project lead: ${projectLead}")

// Docs for this endpoint: https://developer.atlassian.com/cloud/jira/platform/rest/#api-api-2-issue-issueIdOrKey-watchers-post
def result = post("/rest/api/2/issue/${issueKey}/watchers")
        .header('Content-Type', 'application/json')
        .body("\"${projectLead}\"")
        .asString()