This can be done in the following order:
1. Create a boolean report parameter in your SSRS Report.
2. On the textbox which is to be hiden, right click on it and click on propreties.
3. From there click on Visibility tab.
4. In it select expression and put the following expression:
=Iif(Parameters!Report_Parameter_0.Value = true, True, False)
Replace Report_Parameter_0 with the name of the parameter you declared in the report.
5. Now from the web page, set this variable and it will work for you.
Hope this answer your question.