Owning your own home

Probably exterior silicone sealant, surprised the window fitters didn't use some originally.

I would use masking tape either side of the joint (not too close to joint otherwise you will get a ridge when you remove it) apply a thick bead and use a smoothing tool for a nice neat bead.

I’ve just fixed the exact same problem on the front of my pebbledashed house, using the below sealant.

Dried very quickly. Had to use three tubes as the gaps just eat it up.

@mespilus recommended Soudal fix all. I tired it. Did the job.

Fill the gap with backer or expanding foam if required - sealant laid on too thick will not cure.