Buy glazing sealant
Glazing sealant is available in different brands, types and colors. You do have to know a little bit which sealant you need. It would be annoying if you order a type of glazing sealant that cannot be painted over when this is the intention. Or that you just need a special type of sealant to mount windows as used to be done with putty.
We supply glazing sealant in tubes or sausages. The tubes fit in a normal caulking gun. For a sausage you need a special caulking gun that is often only used by the professionals.
It is also useful to choose a color that resembles the painting of your window frames. it is not nice to use a white sealant if you have black frames. It is of course better to opt for a black adhesive.