Moving charge produces current. Therefore if two electric charges separated by some distance and stationary with respect to each other are moving together in some direction, they will constitute currents in same or opposite directions and these currents will interact with each other using magnetic field and they will attract or repel each other using magnetic force. Now this magnetic force between two moving charges depends on the speed of the movement of charges because higher the speed of charges, higher the amount of current and therefore higher amount of magnetic interaction between charges. And higher amount of magnetic interaction leads to higher amount of magnetic force between charges. Hence if Einstein is right that nothing is absolute or absolute reference frame does not exist, we will be able to find two different speeds of the charges with respect to two different and distinct reference frames which will imply two different value of (amount of) magnetic force between the two moving charges at the same time. This is not possible because the force must have only one exact value at a time. It cannot have multiple values at the same time. Therefore Einstein cannot be right in this matter also. This fact that the magnetic force existing between two moving charges has only one value implies that the speed of the charges should have also only one value and that value should be the value of speed with respect to absolute reference frame and not any other speed measured with any other reference frame.
The moving charges described in above paragraph are stationary with respect to each other but both charges are moving with a certain speed with respect to absolute reference frame and hence this movement of charges with respect to absolute reference frame constitutes currents and results in magnetic interaction between them and they experience magnetic force. Since this magnetic force must have only one value, the speed must have only one value which is measured with respect to absolute reference frame only and the speed measured with other reference frame will be different but that speed cannot be used to calculate the magnetic force. Only the speed with respect to absolute reference frame can be used for that purpose. Hence the absolute reference frame is distinct in this way from all other reference frames.
Therefore from above discussion we can define absolute reference frame as a reference frame in which the electrical charges do not have magnetic interaction with each other and magnetic force between charges do not exist because the charges are stationary and not moving and hence they do not constitute any currents. In other words, an object residing in absolute reference frame has zero velocity/speed and zero acceleration as well as zero kinetic energy.