The easiest thing is to set a Default for the user / session (using the Defaults button). 

If that doesn't work for your business needs, scripting can help... depending on details.  (Ex. add a pop-up reminder when the user clicks off the Header tab, and the warehouse is still 000...).

How would I go about scripting... where would I do this?? Can you point me to right place..

User Defined Scripts are added through the Custom Office module.

First you need the script... if you haven't done one before, perhaps someone will be generous to write it for you.  The logic would not be complex (get the warehouse code, then pop-up a messagebox when 000... perhaps adding an extra filter to not do a pop-up when exiting SO entry with no selected order).

Then attach the script to an event, in this case I'd suggest a UI event: Panel - On Exit, for the SO PHEADER panel.

How do I find what panel a script is located... below is a script that we use when a Service Tech code is required.. It looks like a box displays with error stating Service Tech is required..  Wouldn't this script be close to what I need.. The warehouse box would just state on exit .. "Make sure warehouse code is correct" 

retVal = ""
tmpVal1 = ""
tmpVal2 = ""
tmpVal3 = ""

retVal=oSession.AsOject(oSession.UI).Messagebox("Script Fired - test")

'retrieve servicetech1 value
retVal = oBusObj.GetValue("UDF_ServiceTech1$",tmpVal1)

'retrieve salesorderno value
retVal = oBusObj.GetValue("salesorderno$",tmpVal2)
tmpVal3 = left(tmpval2,1)

if tmpVal1 <> "" then
Exit Sub
end if

retVal=oSession.AsOject(oSession.UI).Messagebox("tmpVal2 = " & tmpVal2 & "tmpval3 = " & tmpval3)

if tmpVal3 = "J" then
retVal=oSession.seterror("Service Tech Code is Required")
end if

Thanks for ALL your help.. 

Hello again.. where is the best place to learn to script... is this scripting strictly for Sage?? 

jepritch has a scripting guide posted somewhere which is an excellent reference.

For guided training, I took a course by Alnoor_C back in 2015 and would highly recommend Alnoor's training.

Looking in the BOI forum here can get you some more examples / assistance.

FYI here is a link to the thread that has the scripting document.  

https://www.sagecity.com/us/sage100_erp/f/sage-100-personalization-customization-and-productivity-tools/98736/scripting-helper-documentation

Happy Scripting!