IF statement in URL hack - what am I doing wrong

Question

I have a URL hack which works fine, but I now need to insert an IF statement in to check for a null condition and use another field if so. What am I doing wrong here?

/500/e?
CF00NO00000029qLz={!Order.OrderNumber}
&CF00NO00000029qLz_lkid={!Order.Id}
&retURL={!Order.Id}
&RecordType=012O00000009PQf
&ent=Case
&cas4={!Account.Name}
&cas3={!IF({!Order.Therapist__c}=NULL,{!Order.CustomerAuthorizedBy},
{!Order.Therapist__c})}
&CF00NO0000002MGH0={!Order.EndUser__c}
&00NO00000029qMO=Customer
&00NO00000029qr2=1

Answer 1

Simply use BLANKVALUE function like this below in code :

BLANKVAUE: Determines if an expression has a value and returns a substitute expression if it does not. If the expression has a value, returns the value of the expression.

/500/e?
CF00NO00000029qLz={!Order.OrderNumber}
&CF00NO00000029qLz_lkid={!Order.Id}
&retURL={!Order.Id}
&RecordType={!$Setup.CustomSettingName__c.CustomFieldName__c}
&ent=Case
&cas4={!Account.Name}
&cas3={!BLANKVALUE(Order.Therapist__c,Order.CustomerAuthorizedBy)}
&CF00NO0000002MGH0={!Order.EndUser__c}
&00NO00000029qMO=Customer
&00NO00000029qr2=1

Also, do not hardcode recordtype ids.

Rather create a Hierarchy Custom Setting and access the value like this

RecordType =  {!$Setup.CustomSettingName__c.CustomFieldName__c}

Answer 2

Pretty sure you just need to add in a couple of quotes to make the value being tested a string parameter to the IF():

IF("{!Order.Therapist__c}"=NULL,"{!Order.CustomerAuthorizedBy}",