Example for your vertical line, this one draws at 9:00 a red line:

// --- variables
// --- extern
//Time1=090000   // when, must be format hhmmss
//Red=255        // red part of the RGB color code
//Green=0        // green part
//Blue=0         // blue part
//Visability=255 // alpha value of color 0=invisible 255=fully visible

// check color values do not exeede allowed values
Red=max(Red,0)           // take the highest between users choice and 0
Red=min(Red,255)         // take the lowest between users choice and 255
Green=max(Green,0)
Green=min(Green,255)
Blue=max(Blue,0)        // if user pushes -2 -> take 0 which is higher than -2
Blue=min(Blue,255)      // if user pushes 400 -> take 255 which is lower than 400
Visability=max(Visability,0)
Visability=min(Visability,255)

IF time=Time1 THEN       // if the time is the one of Time1
// syntax: DRAWVLINE(x-AxisValue) coloured(R,G,B,A)
//         x-AxisValue: barindex at that candle, barindex-1 one candle before
//         optional addition coloured() with RGB and optinal Alpha values
// https://www.prorealcode.com/documentation/drawvline/
// https://www.prorealcode.com/documentation/coloured/
DRAWVLINE(barindex) coloured(Red,Green,Blue,Visability)
ENDIF
// --- each indicator must end with some kind of RETURN (but only ONE Return command is allowe)
//     either you plot a line with RETURN VariableName as "a human understandable description"
//     or standalone if you used only DRAW... components
// https://www.prorealcode.com/documentation/return/
RETURN

I attach the indicator file, so you can examine how extern variables are done. Even if you don’t understand any French, you might want to have a look here https://www.prorealcode.com/courses/premiers-pas-avec-la-programmation-pour-prorealtime/

Hope this will help you for the beginning.