Here is what you can try and your work will be done.
Use the command mentioned below to generate a token.
docker swarm join-token manager
This will generate a token for you which you have to copy and paste on those machines which is to be added as Swarm manager.
docker swarm join \
--token SWMTKN-1-61ztec5kyafptydic6jfc1i33t37flcl4nuipzcusor96k7kby-5vy9t8u35tuqm7vh67lrz9xp6 \
By pasting this token over those machine, you'll be able to add them as manager.
I hope the above information will resolve your query.