DO MOUSE statement (i'm not an advanced coder)

I've been trying to create a paint program in true basic, and I don't know how to do something when the mouse equals a certain coordinate. My code for the drawing program is:

CALL Read_image("JPEG",image$,"paintprog.jpg")

LET x=0
LET y=0
BOX SHOW image$ at x,y

DO
LET write$ = "*"

GET MOUSE x,y,s

DO WHILE s = 1

GET MOUSE x,y,s

PLOT TEXT, AT x, y: write$

LOOP
LOOP

END

How do I add another statement that changes the write$ to "." when the cursor is clicked at a certain coordinate?

I know it is similar to either IF or DO WHILE s = 1 AND x >= 120 etc

Comments

RE Do mouse statement

I believe that if you were to say, for instance: IF X=5 AND Y=5 THEN LET WRITE$="." it would work.