Modify the href attribute of tag dynamically in WordPress

Try this (pure JavaScript):

var allBtns = document.getElementsByClassName('nectar-button');

function preventDefaultOpenCustomUrl(event)
{
    event.preventDefault();
    var price = document.getElementById("fieldname9_1").value;
    var url="https://na2.docusign.net/member/PowerFormSigning.aspx?PowerFormId=df6fbf3d-6f5d-4c48-8965-f4fa810099f4&Institutional_Buyer_AnnualPrice=" + price;
    window.open(url);
}
;

for (i = 0; i < allBtns.length; i++) {
    allBtns[i].addEventListener("click", preventDefaultOpenCustomUrl);
}

this is adding the event to all buttons with that classname,
with Jquery:

(function ($) {
    $('.nectar-button').on('click', function (e) {
        e.preventDefault();
        var price = $('#fieldname9_1').val();
        var url="https://na2.docusign.net/member/PowerFormSigning.aspx?PowerFormId=df6fbf3d-6f5d-4c48-8965-f4fa810099f4&Institutional_Buyer_AnnualPrice=" + price;
        window.open(url);
    });
})(jQuery);

if you dont have access to the HTML to add an ID you can use more tags/classnames in the selector to select that specific button.