Setting "read only at run-time" properties??

I am trying to develop a text box user control for my app. I would like to be able to set multiline to true at design-time. The Multiline property is read only at runtime. How do I design the user control so I can set the multiline property ar design-time?

Anytime I try to put code in the control that has Let Multiline and try to assign textbox.multiline = , I get the message this property cannot be set at run-time.

Thanks in advance.