Before you get started, enter a number under the Mesh branch for the Default Element Size so that you get at least 4 elements across the center solid cylinder. The mesh you show is too coarse.
On the Details window for Internal Heat Generation, click the box to the left of the word Magnitude. That will put a P in the box. On the Details window for the Temperature result, click the box to the left of the word Maximum. That will put a P in the box. You have now created a Parameter Set with an input and an output.
There are two ways to proceed from this point.
One approach is in Workbench, open the Parameter Set and type several rows of internal heat generation values in the Table of Design Points, then click Update All Design Points and wait for ANSYS to compute the temperatures. Plot the data and see where the curve crosses the 300K level of temperature. You can enter addtional values to the table if you want to get closer to the cross over value. A good guess for a better value is simply 2E12*300/374.
Another approach is in Workbench, Drag Direct Optimization out of the toolbox and drop it under the Parameter Set. You can setup an Optimization to Seek Target and type in 300 as the target and assign a tolerance of say 1. The optimizer will solve the problem multiple times, automatically adjusting the internal heat generation until it finds the value that gives the desired result.