When a grounding strap is not available, a technician can ground by touching the computer’s:

An antistatic floor mat or ground mat is one of a number of antistatic devices designed to help eliminate static electricity. It does this by having a controlled low resistance: a metal mat would keep parts grounded but would short out exposed parts; an insulating mat would provide no ground reference and so would not provide grounding. Typical resistance is on the order of 105 to 108 ohms between points on the mat and to ground.[4][5][6] The mat would need to be grounded (earthed). This is usually accomplished by plugging into the grounded line in an electrical outlet. It’s important to discharge at a slow rate, therefore a resistor should be used in earthing the mat. The resistor, as well as allowing high-voltage charges to leak through to earth, also prevents a shock hazard when working with low-voltage parts. Some ground mats allow you to connect an antistatic wrist strap to them.