It's hard letting them go.
Its at this age you have to ask yourself, have you brought her up to be street smart? Is she savvy? Do you trust her? Is she responsible? Does she respect boundaries and kerfews? Does she know how to keep herself safe?
At 16 she is legally allowed to have sex, she's also legally allowed to leave home. Although as a parent its your responsibility to keep her safe until she's 18. (NSPCC website).
She will become secretive if you start to (in her eyes), be unreasonable. She will become sneaky and go against you. Also remember, she's a teenager who's going to rebel, it's a common thing that teenage girls and their mums have a push pull battle at this stage.
She's becoming more independent, and she needs to see that you trust her and her decisions.
Use this time to talk to her about how much you love her and that you respect she's becoming a young adult, you have to let go of the control and give your daughter some control.
Let her go, but tell her to message you when she's there, with a selfie of herself and this new boyfriend. Tell her to be back at a certain time. And trust her.
Be prepared to drop everything and go pick her up if things don't work out though!
This is the stage that's the hardest, and for you as a parent you need to help her to navigate growing independence, and foster mutual trust and respect.
The minute you dig your heels in, she will also dig her heels in and conflict will be inevitable.
Good luck op, I'm rooting for you and your daughter, and trust me when I say I speak from experience.