Sinusoids and Phasors AC circuit impedance
The usual way to find a Thevenin equivalent for a circuit with dependent sources is to connect a 1 amp test source to the input. Find the voltage across the terminals, then divide by 1 amp to get the Thevenin impedance:

$$R_{th} = \frac{v_{in}}{1\ A}$$

The Thevenin voltage is found in the usual way, by determining the open-circuit voltage \$v_{in}\$. Without an independent source, the dependent source is inactive. (No current means \$v = 0\ V\$). So the open-circuit voltage is 0 V. The Thevenin equivalent is just an impedance.