Be fully aware that this option is very insecure as simply viewing the HTML source will reveal the user's credentials.
First Step
If what you want is to pass credentials (demouser/demouser) to the Flash control declared inside object/embed tags during development you can modify the noscript section and several other places where you pass in flash vars using JavaScript in the html-template/index.template.html file inside your Flex builder project -
<object classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000"
id="${application}" width="${width}" height="${height}"
codebase="http://fpdownload.macromedia.com/get/flashplayer/current/swflash.cab">
<param name="movie" value="${swf}.swf" />
<param name="quality" value="high" />
<param name="bgcolor" value="${bgcolor}" />
<param name="allowScriptAccess" value="sameDomain" />
<param name="http_user" value="demouser" />
<param name="http_password" value="demouser" />
<embed src="${swf}.swf" quality="high" bgcolor="${bgcolor}"
width="${width}" height="${height}" name="${application}" align="middle"
play="true"
loop="false"
quality="high"
allowScriptAccess="sameDomain"
type="application/x-shockwave-flash"
flashVars="http_user=demouser&http_password=demouser"
pluginspage="http://www.adobe.com/go/getflashplayer">
</embed>
</object>
When deploying you would need to wrap the generated HTML wrapper file (usually in your bin-debug/ folder in a JSP page and use scriptlets or JSTL tags to write out the credentials on a per user basis). So for instance,
flashVars="http_user=demouser&http_password=demouser"
would become
flashVars="http_user=<%=username%>&http_password=<%=password%>"
Second Step
In your Flex code, retrieve the username and password via:
import mx.core.Application;
var username:String = Application.application.parameters["http_user"];
var password:String = Application.application.parameters["http_password"];