<button class="fontButton" onClick="addField()">&plus;</button>
<form id="tasksDataForm"></form>

<script>
var f = document.getElementById("tasksDataForm");
function addField() {

            var tasksDataFields = `<br/> <input type="text"> <br/>`;
<!-- I have to put this button to end of the form and all fields that are added to form should be inserted above it -->
if(tasks.innerHTML == ''){
            tasks.innerHTML += '<input type="submit" value="Save">';
        }

        tasks.innerHTML += tasksDataFields;
    }

</script>