Executing a SQL Stored procedure from a SSRS text box item - Is it possible
I think this used to be easier when SQL Server included SOAP/web services with SQL 2005/2008. With SOAP you could create a wrapper to a SQL web service that would call a stored procedure with a URL. Microsoft gutted that functionality in future releases of SQL Server 2012 and above. I miss the functionality.

I can think of three ways to handle this but there's probably more:

First create an ASP.NET web service 

Here's a tutorial to get you to connecting to a database and accessing the web service with HTML. So basically, you could call your stored procedure from anywhere--not just SSRS.

https://docs.microsoft.com/en-us/aspnet/web-api/overview/older-versions/build-restful-apis-with-aspnet-web-api#Exercise3

And, second, embed custom code with SSRS (here's a few examples I thought were useful):

http://www.davidgiard.com/2012/09/07/EmbeddedCodeInAnSSRSReport.aspx

https://social.msdn.microsoft.com/Forums/sqlserver/en-US/cdf290fb-153d-4bc2-8964-8b550df7a087/calling-stored-procedure-from-custom-code?forum=sqlreportingservices

Last, create an assembly to call a custom function (that calls your procedure):

http://davidgiard.com/2012/09/11/CallingAnExternalAssemblyFromSSRS.aspx