XML : how to evaluate variable in xpath using How.find?

I am using the below syntax to evaluate the Xpath in run time

  [FindsBy(How = How.XPath, Using = "//input[@id='icombobox_Text_cboRefPriority']")]  public IWebElement txt_RefPriority { get; set; }    

the above works perfectly , however in case when i need to select a xpath using the variable it does not work.

e.g :

  Context.LoginName => will read a value from a XL/XML which can change dnamically    [FindsBy(How = How.XPath, Using = "//input[@id='"+Context.LoginName+"']")]  public IWebElement txt_RefPriority { get; set; }    

How to resolve the problem ?