Display portfolio sub pages in navigation menu dropdown

Site URL: https://www.oceanistanbul.com/

I built a portfolio page with sub-pages. Everything works well, but I want to be able to navigate to each of the sub-pages from the navigation menu at the site header by hovering over the portfolio's link and selecting the sub-page title.

You can use anchor links to accomplish this. 

https://support.squarespace.com/hc/en-us/articles/207842357-Adding-Index-Page-anchor-links

I'm sorry, my question wasn't clear I think. I want to do it with a dropdown at the navigation bar.

For example, the navigation bar would have:

• About Us
• Services
• Contact

And once you hovered over Services, it would have a dropdown of 5 different services that are currently subpages in my portfolio.

 Create a folder on the Nav bar. Then put links to the subpages in the folder.  There are some limitations, you can't put a folder in folder. The links would be created using the url slug of those sub pages.  Check out my site, I am doing this already with two drop down menus. Just click stories or pictures to get to my main nav bar then you will see the  drop down menu "special stories" and "photography lessons". I use these drop downs to get to specific blog posts. I also use summary blocks and single links (not in a folder) for specific posts as well. Folders are created the same way as a new page, and links are too. You drag links into the folder, which is on the nav bar,  so they don't appear on the nav bar by themselves. 

https://support.squarespace.com/hc/en-us/articles/206232858-Adding-links-to-your-site

my random views

The link solution doesn't work for me because I don't want one navigation item for the folder and one for the actual portfolio that I built so far. and if I disable the portfolio, then my links don't work. So at this point, I'm trying to find a way to somehow copy my portfolio subpages' content out of them to a new standalone page since that would probably be the easiest.

You can create the folder with sub page links and give them and the folder any name you want, doesn't  need to be the same as the portfolio/index page name, so you don't have duplicate names on the nav bar,  it is the only real way to create a drop down menu, regardless of where you want to link to. That is what a folder is for. This will explain it, and index pages work like portfolio pages. 

https://support.squarespace.com/hc/en-us/articles/205813748-Folders-vs-Index-Pages

Right, but I don't want "Hizmetlerimiz" to show up in the navigation bar above. My understanding is that to do that, I need to disable the page for Hizmetlerimiz portfolio. When I do that, subpages of the portfolio are also inaccessible.

Any page you don't want to show up on the Nav bar, move to the unlinked section of your site. It is still active but can't been seen unless a link takes the viewer there. My blog page is in the unlinked section of my site and links in a folder on my Nav bar are how viewers get to many posts there.  In your case your drop down menu, which is created in the linked section so it is on the nav bar and contains the links,  will take viewers to any page or sub page you make a link to.