Find the common element in two int arrays

I am preparing for a interview for a junior level c# position. I take two arrays and find the common int between both of them. Let me know if you have any feedback.

using System;
using System.Collections.Generic;

namespace CommonElement
{
    class MainClass
    {
        static void EnterArray1IntoHashSet (int[] array1, HashSet hs){
            foreach (int num in array1)
            {
                if (!hs.Contains (num)) 
                {
                    hs.Add (num);
                }
            }
        }

        static int FindCommonInt(int[] array2, HashSet hs){
            foreach (int num in array2)
            {
                if (hs.Contains (num)) 
                {
                    return num;
                }
            }
            return -1;
        }

        public static void Main (string[] args)
        {
            int[] array1 = new int[7]{ 5, 6, 7, 8, 1, 2, 3 };
            int[] array2 = new int[7]{ 13, 22, 3, 45, 67, 73, 85};
            HashSet hs = new HashSet ();
            EnterArray1IntoHashSet (array1, hs);
            int commonElement = FindCommonInt(array2, hs);
            if (commonElement != -1)
            {
                Console.WriteLine (commonElement);
            } 
            else 
            {
                Console.WriteLine ("No Common Element");
            }
        }
    }
}

Improving EnterArray1IntoHashSet

Why do you pass a hash set to EnterArray1IntoHashSet ?
It would be better to let the method create, populate and return the hash set.

You don't need to check if an element is inside a hash set before adding it:
if the element already exists, it will be simply ignored.

The name of this function is unnecessarily tedious.
It has a nice general purpose,
and you can name it accordingly.

static HashSet ToHashSet(int[] array1)
{
    HashSet set = new HashSet();
    foreach (int num in array1)
    {
        set.Add(num);
    }
    return set;
}

Coding style

The placement of opening braces { is inconsistent:
sometimes you put it on the next line,
sometimes on the same line.
The .NET convention is to put it on the next line.

It's also unusual to put a space between method name and the ( when calling them, for example this:

Console.WriteLine (commonElement);

Just omit the unnecessary space:

Console.WriteLine(commonElement);

StackExchange Code Review Q#100371, answer score: 7