Create a parameter or variable, let's say filePath, to store the entire file path. You can set its value using an expression.
If you're using a parameter, you can use the following expression in the parameter's value field:
@concat(dataset().folderPath, '/', dataset().fileName)
If you're using a variable, you can use the Set Variable activity and set the variable's value with the expression: @concat(dataset().folderPath, '/', dataset().fileName). Here, dataset().folderPath and dataset().fileName are system variables provided by Azure Data Factory that represent the folder path and file name, respectively, of the current file being processed.