Subtext is not a technique applied to dialogue. It is a structural consequence: if a character wants something they cannot ask for directly, everything they say is about it and none of it says it. Get the want and the prohibition right and subtext appears on its own. Write the want with no prohibition and no amount of oblique phrasing will produce it.
The most reliable generator in a scene is a rule about what cannot be said — professional, social, or personal. Two colleagues who cannot mention the redundancy list, two friends who cannot mention the money, a couple who cannot mention the appointment. The scene can then be about anything at all and will still be about that.
The audience does not need to decode subtext consciously to feel it. What they register is that the scene is denser than its surface, which is the difference between dialogue that reads as written and dialogue that reads as overheard.
What is subtext in a script?
The meaning underneath the literal words, created by the distance between what a character wants and what they are permitted to say. It is a structural result of a prohibition, not a style of writing.
How do you write subtext?
Give the character something they want from the other person and a reason they cannot ask for it. Then write the scene about anything else. The prohibition does the work.