How do I insert primarykey value as foreign key in other table using webapi with ENTITY FRAMEWORK?

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C#

ASP.NET

i tried to insert primary of one table value to table as foreign key value but when i tried to fetch the data of other table where value is stored as foreignvalue iam getting internal server error response is not created .
which one is the right process to fill foreignkey value when we code using webform c# with http verbs by consuming webapi service with entity framework .

What I have tried:

this is my primary key value table data

 public class ADDCATEGORYCLS
   {
       [System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage", "CA2214:DoNotCallOverridableMethodsInConstructors")]
       public ADDCATEGORYCLS()
       {
           this.VEL_ADDROOM = new HashSet<ROOMCLS>();
       }
public int CATEGORY_ID { get; set; }

[System.Diagnostics.CodeAnalysis.SuppressMessage("Microsoft.Usage","CA2227:Collectio,PropertiesShouldBeReadOnly")]

public virtual ICollection<ROOMCLS> VEL_ADDROOM { get; set; }

this is foreign key value table code

  public class ROOMCLS
    {
 public Nullable<int> CATEGORY_ID { get; set; }
        public virtual ADDCATEGORYCLS VEL_CATEGORY { get; set; }
}