How to create custom service v4 my todo project?

You can try something like :

 
      const { user } = ctx.state;
      const { data } = parseMultipartData(ctx);
      const sanitizedInputData = await this.sanitizeInput(data, ctx);

      const entity = await strapi
        .service("api::todo.todo")
        .create({
          data: { ...sanitizedInputData, user: { ...user } },

        });