If the package provides a method to specify the option to use the EmailHeader.triggerAutoResponseEmail flag, use it. Otherwise, one of the two following triggers should probably work:
trigger AfterLeadInsert on Lead (after insert) {
Lead[] newLeads = [SELECT Id FROM Lead WHERE Id = :Trigger.new];
Database.DMLOptions autoResponseOptions = new Database.DMLOptions();
autoResponseOptions.triggerAutoResponseEmail = true;
Database.update(newLeads, autoResponseOptions);
}
trigger AfterLeadInsert on Lead (after insert) {
Lead[] newLeads = [SELECT Id FROM Lead WHERE Id = :Trigger.new];
Database.DMLOptions autoResponseOptions = new Database.DMLOptions();
autoResponseOptions.triggerAutoResponseEmail = true;
for(Lead record: newLeads) {
record.setOptions(autoResponseOptions);
}
Database.update(newLeads);
}
I provide both options here, because sometimes one or the other doesn't seem to behave as documented, so you'll need to test. You'll probably also want to limit this code to just certain lead sources or something specific, because as written, this will always send an email, even for manually created leads.
This code was adapted from this help article.