https://webstool.ru/dinamicheskoe-pole-emailsubject-v-formit.html
короче оказалось все очень просто, там напрямую можно указывать значение полей:
[[!AjaxForm?
&snippet=`FormIt`
&form=`formEmailProduct`
&emailTpl=`mailTplFEmailProduct`
&hooks=`email`
&emailSubject=`[[+subject]]`
&emailTo=`[[+emailTo]]`
&emailFrom=`[[+emailFrom]]`
&validate=`name:required, emailTo:required, emailFrom:required, subject:required`
&validationErrorMessage=`Fill in all fields!`
&successMessage=`The application has been sent successfully, you will be contacted soon!`
]]