validate blank textbox in vba

how to validate text box if Combo1 have value is "OTHER" then textbox1 should be validate otherwise not. 

Combo box have department 

if their is not registered department in combo then user will select OTHER at that time textbox will enable for manually enter department name. 

if tuser not enter any value in textbox then error message should be display.

if enter than should be save record.

how to do this ?