This is small addon for mark target using a command.

Note: By default marker is "White Skull"

Main command works in group and raid only and if your group role is "Tank" or "Main Tank" in the raid:

/automarktarget

For only solo play use the command:

/automarktarget_solo

For any (solo, group, raid and does not check your group role) use: 

/automarktarget_any

Simple solution add it command to the attack macro, and your group will always see your are current target.

For example:

#showtooltip Charge
/startattack
/cast Charge
/automarktarget

Also may use another marker:

• Yellow 4-point Star
  

  /automarktarget star

   

• Orange Circle
  

  /automarktarget circle

   

• Purple Diamond
  

  /automarktarget diamond

   

• Green Triangle
  

  /automarktarget triangle

   

• White Crescent Moon
  

  /automarktarget moon

   

• Blue Square
  

  /automarktarget square

   

• Red "X" Cross
  

  /automarktarget cross