BGP prefix metric

Hi!

Sorry for my bad English.
I need to setup BGP from Mikrotik to one standalone provider. It will be my default.
I like to announce /24, which is part of /21 network, what was announce yet from other hardware on our company.

How i can announce /24 prefix from mikrotik, and for that this announe was HIGH priority ?

I need that my /24 announce will be most priority, and traffic from world go to this network for this link.
I think that i need some metric setup? But cant find where and how i can regulate this.

Thanks