voglio autenticarmi tramite una pagina php su un form di una pagina asp presente su un sito esterno

il form a cui vorrei autenticarmi e composto in questo modo

Codice PHP:
        <form name="aspnetForm" method="post" action="default.aspx" id="aspnetForm">
<
input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="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" />

<
input name="ctl00$Login1$tbxUserNameid="ctl00_Login1_tbxUserName" class="with100" type="text">         
<
input name="ctl00$Login1$tbxPasswordid="ctl00_Login1_tbxPassword" class="with100" type="password">         
<
input name="ctl00$Login1$btnLoginHomeid="ctl00_Login1_btnLoginHome" src="imgs/commons/btn_form_enter.png" style="border-width: 0px;" align="top" type="image"
ho provato le varie sluzioni presenti sul forum ma sembrano non funzionare