Use Unity3D's EventSystem to set or unset focus on a UI element

Unity3D_InputFieldSetFocus.cs

public TMP_InputField inputField;
float focusReleaseTime;
float focusReleaseCooldown = 0.1f;

public bool HasFocus () {
    return EventSystem.current.currentSelectedGameObject == inputField.gameObject;
}

public void SetFocus (bool value) {
    if (value && !inputField.isFocused) {
        inputField.ActivateInputField ();
        ToggleVisibility (true);
    } else {
        focusReleaseTime = Time.time;
        inputField.DeactivateInputField ();
        if (!EventSystem.current.alreadySelecting) {
            EventSystem.current.SetSelectedGameObject (null);
        }
        ToggleVisibility (false);
    }
}

void ToggleVisibility (bool visible) {
    // ..
}

void Start() {
  ToggleVisibility (HasFocus ());
}

void Update () {
    // use the hardcoded RETURN key for starting and submitting a chat message
    if (Input.GetKeyDown (KeyCode.Return) && Time.time >= focusReleaseTime + focusReleaseCooldown) {
        // set focus if we don't have focus
        if (!HasFocus ()) SetFocus (true);
        // but if we have focus with no text then we should release focus
        else if (inputField.text.Length == 0) SetFocus (false);
    }
}