Perguntas Validando campo customizado

Pessoal, sou super iniciante em ADVPL e estou precisando colocar uma validação de preenchimento de campo em uma tela que eu criei. Alguem pode me ajudar?

DEFINE MSDIALOG oDlgInfo TITLE "Informações Coleta" FROM 000, 000 TO 280, 400 COLORS 0, 16777215 PIXEL
@ 002, 002 SAY oSay1 PROMPT "Motorista" SIZE 025, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 010, 002 MSGET oMotoris VAR cMotoris SIZE 195, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 025, 002 SAY oSay2 PROMPT "Documento" SIZE 035, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 032, 002 MSGET oDoc VAR cDoc SIZE 060, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 025, 068 SAY oSay3 PROMPT "Placa" SIZE 035, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 032, 068 MSGET oPlaca VAR cPlaca SIZE 060, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 047, 002 SAY oSay4 PROMPT "Transportadora" SIZE 050, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 055, 002 MSGET oTransp VAR cTransp SIZE 195, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 070, 002 SAY oSay5 PROMPT "Observação" SIZE 050, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 077, 002 GET oObs VAR cObs OF oDlgInfo MULTILINE SIZE 195, 044 COLORS 0, 16777215 HSCROLL PIXEL
DEFINE SBUTTON oSButton1 FROM 125, 135 TYPE 02 OF oDlgInfo ENABLE ACTION oDlgInfo:End()
DEFINE SBUTTON oSButton2 FROM 125, 170 TYPE 01 OF oDlgInfo ENABLE ACTION (_nOpc := 1,oDlgInfo:End())
ACTIVATE MSDIALOG oDlgInfo CENTERED

oMotoris:= .T.
if oMotoris == ''
MsgStop("Campo não preenchido")
EndIf

Você pode usar a cláusula valid do seu MsGet

DEFINE MSDIALOG oDlgInfo TITLE "Informações Coleta" FROM 000, 000 TO 280, 400 COLORS 0, 16777215 PIXEL
@ 002, 002 SAY oSay1 PROMPT "Motorista" SIZE 025, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 010, 002 MSGET oMotoris VAR cMotoris VALID !Empty(cMotoris) SIZE 195, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 025, 002 SAY oSay2 PROMPT "Documento" SIZE 035, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 032, 002 MSGET oDoc VAR cDoc SIZE 060, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 025, 068 SAY oSay3 PROMPT "Placa" SIZE 035, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 032, 068 MSGET oPlaca VAR cPlaca SIZE 060, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 047, 002 SAY oSay4 PROMPT "Transportadora" SIZE 050, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 055, 002 MSGET oTransp VAR cTransp SIZE 195, 010 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 070, 002 SAY oSay5 PROMPT "Observação" SIZE 050, 007 OF oDlgInfo COLORS 0, 16777215 PIXEL
@ 077, 002 GET oObs VAR cObs OF oDlgInfo MULTILINE SIZE 195, 044 COLORS 0, 16777215 HSCROLL PIXEL
DEFINE SBUTTON oSButton1 FROM 125, 135 TYPE 02 OF oDlgInfo ENABLE ACTION oDlgInfo:End()
DEFINE SBUTTON oSButton2 FROM 125, 170 TYPE 01 OF oDlgInfo ENABLE ACTION (_nOpc := 1,oDlgInfo:End())
ACTIVATE MSDIALOG oDlgInfo CENTERED


Abraço!