This is a possible solution although it is not the most correct.
Put your combobox in column 0 of the msgbar, or define an item with the width you consider and insert your combobox in that item. The same can be done by inserting your scrollbar in a second item. Note that you probably have to program the behavior of the scrollbar control and resize this control
DEFINE WINDOW oWindow FROM nRow, nCol TO nWAlto + nRow, nWAncho + nCol ;
PIXEL COLOR CLR_WHITE, Rgb( 182, 182, 182 )
oMsg := TMsgBar():New( oWindow, , .F., .F., .F., .F., oWindow:nClrtext, ;
oWindow:nClrPane, , .F., .F., .F., .F., .T. ) //l2010, lBorder, l2015 )
//METHOD New( nRow, nCol, nMin, nMax, nPgStep, lVertical, oWnd, nWidth, nHeight,;
// bUpAct, bDownAct, bPgUp, bPgDown, bPos, lPixel, nClrText,;
// nClrBack, cMsg, lUpdate, bWhen, bValid, lDesign ) CLASS TScrollBar
oHScroll := TScrollbar():New( 1, 250, 0, 100, , .F., oMsg, ;
oMsg:nWidth - ( 250 / 2 ) - oMsg:nHeight * 2 + 2, ;
oMsg:nHeight - 2, , , , , , .T. )