As a general rule, given that the demand for tickets for finals and other important matches tends to be much higher than the number of tickets actually available to the club, the club usually runs a ticket application process using a web based form that is exclusively available to members.
FC Barcelona will announce the dates of these processes by the usual channels once the team has qualified for the final and the location and number of tickets is known. If there is greater demand than there are tickets available then the allocation will be made by means of a draw held before a notary.
Please check the club website in order to receive the very latest information. If there is updated information, you will find the links listed there.
If there is no information on the matter on the website, then that is because the draw and/or sales process has not been defined yet, and so the Barcelona Supporter Services Office will not be able to offer you any further information