User input for registry key.

I need the user to input the which com port they are using during installation so this can be written into the registry.

WriteRegStr HKLM SOFTWARE\svr "ComPort" "com1"

All I would need is for the user to input the number, I.E. "1", "5", etc...

Use InstallOptions to create a custom page. On your custom page, use a text box control with the Flags set to 'Only_Numbers'.

For more info, have a look at the InstallOptions ReadMe doc included with NSIS.