Validate blank (empty) TextBox in VB 6.0

how to validate text box if Combo1 have value is "OTHER" then textbox1 should be validate otherwise not in VB6.0. 

if Combo1.text equal to OTHER then it should be validate otherwise not.

in my program the textbox 1 is disable for enter manual department name if user selecting department name from combo1 then textbox1 should not be validate if user select Combo1.text is OTHER from combobox then textbox1 should be enable and if user leave blank Textbox1 then textbox 1 should be validate for manual department.

how to do this ?