Did I understand correctly, you want to send a postcard to Sweden to get it postmarked and sent it back to you?
Technically you are out of luck here, Swedish post does not automatically postmark the stamps anymore. They do have seasonal postmarks, but they don’t really inform the public about those so you never know if you get one. Also, we don’t have post offices.
However, I did hear rumours few years back that the Swedish philatelist association can postmark cards. I don’t know if that is true, but they will surely know more and might be able to help you. I think your best option is to contact them directly via e-mail and ask (https://filatelisten.se You can find email address neatly hidden in upper left corner) Just be ready for a long wait time. The summer holiday season is just starting here so people are not at office to reply for emails.
Another option is to buy a first day cover from PostNord’s website. It will cost more because one cover includes all the stamps of the same issue, but you will for sure get postmarked stamps.
If you want you can send it to me and I will drop it off, but I can’t promise that it will arrive postmarked.
Thank you for your reply! 
