That's good advice there from Gryd3.
This circuit
may do what you want. It uses a 74HC4066 or CD4066 quad bilateral switch IC to make the connections across the required keys.
View attachment 18130
Keyboard keys are scanned by the microcontroller in a grid pattern. The alternative would be for the microcontroller to have a separate input pin for each key, which would be impractical. So each key must be an electrically isolated "dry contact". These can be mimicked using relay contacts, but if both sides of the switch are known to always be between the positive and negative supply rails, a bilateral switch device can be used, with no moving parts.
The circuit is powered from the +5V and 0V rails of the keyboard. If the keyboard operates at a lower voltage such as 3.3V the circuit should still work but the bilateral switch IC works best at 5V.
The 74HC4066 (preferred) or CD4066 quad bilateral switch IC contains four bilateral analogue switch circuits. These are shown by the graphics inside the IC rectangle. Each switch has a control input, and two "contact" points. When the control input is logic high, the switch conducts (more or less) betwen its contacts; when the control input is logic low, the switch does not conduct.
The pushbutton and R1 produce a voltage at R2 that is high when the button is pressed, and low when it is released, but it has noise during transitions, especially the transition from OFF to ON. This noise is called "bounce" and it is caused by the mechanical nature of the pushbutton. C1 smooths out this bounce by slowing down the signal so it cannot change quickly.
R3, R4, R5 and one switch in U1 (on pins 3, 4 and 5) form a circuit called a Schmitt trigger. This circuit cleans up the slowly changing voltage across C1 and provides a clean, sharp transition at U1 pin 4. Search for Schmitt trigger on Electronics Point or Google to learn about how they work.
The cleaned-up pushbutton signal on U1 pin 4 is connected directly to U1 pin 6, where it controls the switch that you will connect across the modifier key - that is CTRL in your case. That switch therefore closes first and opens first. It is also fed through a brief delay (about 10 ms) implemented by R6 and C2, and to U1 pin 12, where it controls the switch that you will connect across the letter key. This switch therefore closes after the modifier key has closed, and opens after the modifier key has opened.
Pins 8~11 of U1 can be connected directly across the pushbutton or membrane contacts of the keyboard and should not disturb its normal operation.
C3 is a decoupling capacitor for U1 and is required for reliable operation. It should be connected as directly as possible between pins 14 and 7 of the device.
C3 should be a ceramic capacitor. C1 and C2 can also be ceramic capacitors with X7R dielectric, so their values are reasonably stable, or film capacitors, for better timing accuracy.
The circuit can be built up on stripboard.