Sissy has two quite separate meanings. The old one is a playground insult for a boy or man judged weak or unmanly. The newer one names a kink in which a man is feminised as part of a negotiated scene, usually with a dominant partner directing it.
The ordinary meaning
Used plainly, sissy is an insult built on the idea that there is a correct way to be a man and that falling short of it is shameful. It has been used that way for well over a century. Most people who use it today mean it dismissively.
The kink
Sissification, or forced feminisation, takes that shame and plays with it deliberately. It can involve clothing, makeup, manners, names and orders, and it is usually part of a femdom dynamic. The word “forced” in the phrase is theatre: nothing about it is actually forced, and it is agreed in advance like any other scene.
What people get from it varies. Some describe relief at being told what to do. Some enjoy the humiliation element specifically. Some simply like the clothes and the attention, with no humiliation in it at all.
It is not the same as being trans
This confusion is common and worth clearing up. A kink involving feminisation is something a person does in a scene. Being transgender is who a person is, all the time, and has nothing to do with kink. Some people are both and most are neither, but the two are not versions of the same thing. The trans and TS guide covers the latter.
Booking it
Some London escorts who offer domination also offer feminisation, with wardrobe and makeup, and some do not. It needs saying plainly in the first message: what you want, how far it goes, and whether humiliation is part of it or not. Those are three different bookings. Fetish and domination escorts covers how to ask, and the safe word guide covers stopping.