How to select and pass workbook names that are open in different instance?

I'm trying to develop an excel file that contains VBA to manipulate and adjust repots that are excel files generated from SQL Server using Excel-Macro Enabled Template. The excel that contains the VBA has a List Box to shows all excel files that are open and a text box for the selected report, as shown in the attached picture. The 1st problem I faced was to display excel report that are open in a different instance of Excel. I posted a question and I got an answer, attached link, my previous question, that solved the problem and open excel files can be shown in the List Box. The problem now is, how can I pass the name of the selected report (workbook name) to subs and functions to become the file that will be manipulated using the VBA code? I tried this way Sel_Rpt = Sheet1.TextBox1.Value and then Set Rpt = Workbooks(Sel_Rpt).Worksheets("Report"), but it didn't work.