There's isn't a command where you can input while a loop is running. However, there are ways to solve this. This is the way I do it:
string$ = ""
oldstring$= ""
do
text 0,0,string$
`input
if oldkey$ <> inkey$()
if asc(inkey$()) <> 8
string$ = string$ + inkey$()
else
if len(string$) > 0
temp$ = ""
for i = 1 to len(string$) - 1
temp$ = temp$ + mid$(string$,i)
next i
string$ = temp$
endif
endif
oldkey$ = inkey$()
endif
sync
loop