com.unity.uiextensions/Runtime/Scripts/Utilities/InputFieldEnterSubmit.cs

/// Credit Vicente Russo  
/// Sourced from - https://bitbucket.org/ddreaper/unity-ui-extensions/issues/23/returnkeytriggersbutton

using UnityEngine.Events;

namespace UnityEngine.UI.Extensions
{
    /// <summary>
    /// Usage: Add this component to the input and add the function to execute to the EnterSubmit event of this script
    /// </summary>
    [RequireComponent(typeof(InputField))]
    [AddComponentMenu("UI/Extensions/Input Field Submit")]
    public class InputFieldEnterSubmit : MonoBehaviour
    {
        [System.Serializable]
        public class EnterSubmitEvent : UnityEvent<string>
        {

        }

        public EnterSubmitEvent EnterSubmit;
        public bool defocusInput = true;
        private InputField _input;

        void Awake()
        {
            _input = GetComponent<InputField>();
            _input.onEndEdit.AddListener(OnEndEdit);
        }

        public void OnEndEdit(string txt)
        {
            if (!Input.GetKeyDown(KeyCode.Return) && !Input.GetKeyDown(KeyCode.KeypadEnter))
                return;
            EnterSubmit.Invoke(txt);
            if (defocusInput)
            {
                UnityEngine.EventSystems.EventSystem.current.SetSelectedGameObject(null);
            }
        }
    }
}