Have a look at this post too
https://www.outsystems.com/forums/discussion/71163/associate-sales-certification/#Post288925
"The sales certification visible only with Partner center.
it is not showing in your profile."
This means you are able to do the pre-sales certification if you are in the partner center. If you are not, its not possible, just if you ask for the company where you are working with.
But of course, I am not sure because I never tried with or without being in a partner center, but if there is no answers I would open a support ticket in support and ask how to take how long can I have the sales certification etc.
