Can I use a custom action to open a form and populate a lookup field?

Can I use a custom action to open a form and populate a lookup field?

I'm using a report with a custom action, trying to open up a form and populate a lookup field with a project code but having issues.

My custom action function is as follows:
  1. void display.BookAppt(int id, Add_Project proj)
  2. {
  3.     openUrl("#Form:Book_Appointment?Project=" + proj.Project_Code, "same window");
  4. }
On the Book_Appointment form, I have a lookup field called "Project" and I'm trying to pass the "Project Code" value from the Add_Project form, but it isn't working.

if I change the function to pass another variable, e.g. "Full_Name" from the Add_Project form, it works without any problems.

Is this a bug with Zoho Creator, or is there another reason why I can't pre-populate a lookup field with openUrl?

Thank you.