Hi aguai,
create in your form one filed from type Select - Dynamic.Generally I use Query: Free:
SELECT title text, id value FROM #__categories WHERE extension = 'com_content' ORDER BY title
Storage: standard | article | catid
Multiple: No
you can set as default-value now
Remark: For a SearchForm:
with Button 3 (Match + Stage) in your form (SearchForm) you have to choose Words | Any Words exact
best regards
Castor