Error - Text Property is read-only

[The Welshman]

As DataMiser says, Dropdown combo's text can only be derived from the existing list by using the list index property.
Try this sub

Sub UpdateMyCombo(cmb as combobox, mytext as string)
dim i%
for i = 0 to cmb.listcount -1   'If its in the list already use it
If mytext = cmb.list(i) then 
cmb.listindex = i 
exit sub
end if
next i
cmb.additem mytext            'If not add it
cmb.listindex = cmb.listcount -1
end sub