Hello Erik,
Thank you for your answer!
The site I am using to test associate lead JS is on my local machine. Here is the code of test function (with hardcoded email and key):
    _submitClickHandler: function () {
       var email = "milena.knijarova111@telerik.com";
       var key = "30292249c804c3ae86ae80ea097ef9eb56dc02af";
   
      mktoMunchkinFunction('associateLead', {
            Email:email
            //FirstName:"milena2",
            //LastName: 'lastName2',
            //Street:'Street'
           },key
        );
    }
There is no JavaScript errors.
Thank you for your help in advace!