Most current laptops have a Universal Audio Connector on them—
see this Dell specific page. You will need a splitter cable that has a TRRS connector to plug into the laptop and an M-M stereo cable. There really is no 'Line' input on this connector as it is really setup to use an external mic, but you can get away with a loopback connection.
Quit REW if running.
Open the Sound control panel and click on the Recording tab. Highlight External Mic and then click the Properties button. Click the Listen tab and uncheck 'Listen to this device'.
Click the Levels tab and set the Microphone Boost to zero (0.0 dB). Set the level to around 85 to start.
Click the Advanced tab. The single channel mic input is usually going to be limited to 16 bit/48KHz, select that. Uncheck Signal Enhancements. Click Apply and close the recording dialog.
In the Sound control panel, click the Playback tab. Highlight the audio output—might be labelled 'RealTek HD Audio 2nd output' or something similar. Click the Properties button.
Click the Levels tab and set the output to 100.
Click the Advanced tab and set the Default Format to: 2 channel, 16 bit, 48000 Hz. Uncheck the Signal Enhancements checkbox.
Click the Spatial Sound tab and set the format to Off. Click the Apply button and close the dialog.
Back in the Sound control panel, click the Recording tab, highlight the External Mic and click the Properties button. Click the Levels tab. Leave the dialog open.
Start REW and open the Preferences. Select the Java driver. Set the Sample rate to 48KHz. Select the Output Device to match the Sound Control panel Output device. Select the Output as HEADPHONE. Select the channels—usually L+R.
Select the Input Device as External Mic. Select the Input as MICROPHONE. Select the input channel you want to use. Select the Timing Reference Output and Loopback Input channel.
Set the Sweep Level to -12 dBFS.
Set Levels to 'Use main speaker test signal to check/set levels'. Click the Check Levels button. Click Next. The test signal will begin to play.
Switch back to the Sound control panel and adjust the mic level so that it is at or below the Output signal level in REW—you won't get it exactly on as there isn't that kind of resolution in that slider to get you there. The REW Preferences dialog will stay on top of everything else so you will be able to see the meters to set the level. When the levels are set click finish in the REW preferences and the signal will stop.
Close the Preferences and close the External Mic settings and Sound control panel.
You can now run your sweeps.
You should not expect much in the way of performance from the internal sound card. Since the input is designed as a Mic input and not a line-level input, it will be very noisy and the levels of distortion are going to be very high. Although it would cost a significant amount, you probably should get yourself a good quality USB audio interface for this kind of testing.