There are two methods:
% Folder is not a cell string and not the name "pwd":
Folder = 'C:\Users\xxx\Documents\DTU\SAXS\folder1';
[filename, dirname] = uigetfile(fullfile(Folder, '*.txt'), 'select file')
This does not work if you want to provide a list of accepted file extensions. Then you can specify the initial folder as 3rd argument:
Folder = 'C:\Users\xxx\Documents\DTU\SAXS\folder1';
[filename, dirname] = uigetfile({'*.txt', 'Text-files (*.txt)'; ...
'*.asc', 'ASCII files (*.asc)'; ...
'*.*', 'All Files (*.*)'}, 'select file', Folder)
% ^^^^^^
