From the end point of your straight run of pipe, use Draw joint icon to draw offsets at the desired angle. In this example we start by using the Draw join icon to draw 1 joint/point 45 degrees from the end point of the straight run of pipe. Next, click the Draw pipe icon to draw a pipe from the end point of the straight pipe to the newly drawn joint with the "Auto Add Elbows" option switched to 'No' as shown.
Next, click 'Set Line view' icon 
and use the Draw joint icon to draw a joint a short distance away from the end point of the straight run 45 degrees on the other side and select the 2 joints as shown to Assign>Joint>.Joint constraint>Body type to establish a rigid connection between the 2 joints. Now use Draw joint command to draw another joint offset to the end of the other Y leg as shown in the same direction, then use Draw pipe icon to "connect the dots" drawing the pipe on the other side of the Y. Please note that by using Insert commands from the end point of either side of the Y pipe, users can continue adding piping, supports, and components which follow the local axis of the piping using the 'Along length' option in the Insert dialogue.
If this Y pipe is a fitting, it is likely to be heavier and stiffer than the model we just created. Users may consider assigning pipe property modifiers and enter scale factors to increase stiffness, weight, and mass by some percentage. Although the piping codes do not address Y pipes, if the manufacturer provides an SIF, you can select the crotch joint of the Y and use Design menu>Assign Design properties with Point Object type, to assign User SIFs.
