Being a freelancer often means you're constantly trying to sell yourself, and your services. There is a relatively unknown Twitter feature which allows you to write a Tweet with a hyperlink to creating a DM.
It's really simple -- you just need to write a Tweet containing the following link:
<id> is your Twitter ID, retrievable using something like Get Twitter ID.
For the Tweet, I'd recommend keeping it short and sweet, just listing your services itemized with an emoji or two. This makes it attractive and eye-catching. Then, I'd pin it to your Twitter profile so it's the first thing people see when they come across your profile.
You also need to go to the Twitter Safety page and tick the box saying "Receive Direct Messages from anyone".
The result is something like: