Question How to set value on field dynamically with JS

@danihml,

You can use the js code below. Just change the red-marked part in the code: 

add_filter('wpdiscuz_js_options', function($jsOption) {
    if (true) {//condition if user logged in other API and user exist
        $jsOption['other_api_user_login'] = 'user_login';
        $jsOption['other_api_user_email'] = 'user@email.com';
    }
    return $jsOption;
});
add_action('wpdiscuz_front_scripts', function() {
    $wpdz = wpDiscuz();
    $handle = 'wpdiscuz-ajax-js';
    if ($wpdz->options->general['loadComboVersion']) {
        $handle = 'wpdiscuz-combo-js';
    }
    wp_add_inline_script($handle, 'jQuery(document).ready(function ($) {
                                     if(wpdiscuzAjaxObj.other_api_user_login){
                                        $("#wpdcom .wc_name").val(wpdiscuzAjaxObj.other_api_user_login);
                                        $("#wpdcom .wc_email").val(wpdiscuzAjaxObj.other_api_user_email);
                                        Cookies.set("comment_author_email_" + wpdiscuzAjaxObj.cookiehash, wpdiscuzAjaxObj.other_api_user_email);
                                        Cookies.set("comment_author_" + wpdiscuzAjaxObj.cookiehash, wpdiscuzAjaxObj.other_api_user_login);
                                     }
                                   });');
});

Pay attention to the commented part next to the red-marked value. 

thanks for the answer @Asti, but my console is getting an error on that part:

if ($wpdz -> options -> general['loadComboVersion'])

Uncaught SyntaxError: Unexpected token '>'. How that works exactly? 

Here is my code modified for get the value from API:

const userInfo = getLastUserInfo();

add_filter('wpdiscuz_js_options', function ($jsOption) {
	if (userInfo) {
		$jsOption['username'] = userInfo.fullName;
		$jsOption['uid'] = userInfo.uId;
		for (let i = 0; i < userInfo.contactList.length; i++) {
			const mail = userInfo.contactList[i];
			if (documentCPF.type === 'email') {
				$jsOption['usermail'] = mail;
			}
		}
	}
	return $jsOption;
});

add_action('wpdiscuz_front_scripts', function () {
	$wpdz = wpDiscuz();
	$handle = 'wpdiscuz-ajax-js';
	if ($wpdz -> options -> general['loadComboVersion']) { //error part
		$handle = 'wpdiscuz-combo-js';
	}
	wp_add_inline_script($handle,
		`jQuery(document).ready(function ($) {
			if (wpdiscuzAjaxObj.username) {
				$("#wpdcom .wc_name").val(wpdiscuzAjaxObj.username);
				$("#wpdcom .wc_email").val(wpdiscuzAjaxObj.usermail);
				$("#wpdcom .wc_uid").val(wpdiscuzAjaxObj.uid);
				Cookies.set("comment_author_" + wpdiscuzAjaxObj.cookiehash, wpdiscuzAjaxObj.username);
				Cookies.set("comment_author_email_" + wpdiscuzAjaxObj.cookiehash, wpdiscuzAjaxObj.usermail);
				Cookies.set("comment_author_uid_" + wpdiscuzAjaxObj.cookiehash, wpdiscuzAjaxObj.uid);
			}
		});`
	);
});

I inserted the code directly into a .js file that is loaded into the blog pages