Yes you can add your own TXT records to any of your domains DNS. So if you want to add something like Google Search Console verifications etc, then you can do it very easily.

To add a custom TXT record, following these instructions:

1. Click on the site name from the main dashboard view. This will take you though to the sites overview screen.
2. You will tabs along the top, select the TXT Records tab. 
3. Click the ADD NEW + button at the top Right
4. Simply enter your TXT record in the popup and then click the create button. Note - You only need to add the actual record, you do not need to any anything more. It will automatically create the @ record for you.
5. Wait for the new TXT records to be added to your DNS and to propogate - see below for more information on this.

You can see in the screenshot below, the tab metioned in step 2 and the new button mentioned in step 3.

How long does it take for the record to go live?

After adding the record, your DNS records will be queued for submission. There is a lot the system has to do, to keep everything in balance for you. You should allow at the absolute maximum, 24 hours for your new TXT records to be live and active. Usually they will be done in a lot less time. But to ensure they are live and propogating fully, you should always allow 24 hours.