How to generate connection string while program already running

Each user in my case has a special connection string (connects to different database), so I have to generate connection strings while program is already running (data for each user is stored database). Please, don't question this, I know why i do that.

But, how to write it correctly into ASP.NET? When I have connection string stored in Config, i would write ConnectionString="<%$ ConnectionStrings:IOFakeConnectionString %>", but i need something like ConnectionString="<%$ CallForMethodWhichReturnsActualConnectionString %>".

I don't want to change it at Page_Load, like this.SqlDataSource1.ConnectionString = GetConnStr();. This is not the way I want it to work. It should be in primar logic of application.
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