iRule for URI

It seems like what you are asking for is a URI rewrite though its a bit hard to tell with the vague details provided. If you want to have a certain URI direct users to a page where they can select a certificate, then get redirected to the main webpage, I think this iRule will help.

when HTTP_REQUEST
{
    if {[HTTP::uri] eq "/specific/uri/of/page"}
    {
        [HTTP::uri] "/certificate/selection/page"
    }
}

If you explain further, I am certain I can help more.