you will need to customize the default create controller (can be found here: https://strapi.io/documentation/v3.x/concepts/controllers.html#create )
Effectively you will need to loop through the relational data you want to create, check if it exists, else create it and construct an array of IDs to pass along to the create service for the post model.
We don’t by default do this as it’s generally expected you handle this on your frontend side.