Create external link to another workbook cell programmatically

Hello,

WHat is the syntax for creating an external link to a cell in another
workbook where the extternal workbook name is entered in another cell.

For example, if user enters the name of the other workbook into cell
A1='[Portfolios.xls]Sheet1'!, i want to put into cell C1 of another workbook
the value of the contents of the cell B10 in the user-specified workbook but
i want to get the data by referencing the name of the workbook the user
entered in cell A1, instead of referencing the workbook name explicitly.

Thanks for your help.