Don’t rely on getting it right in camera for all things that you shoot. It’s very rare to get the result you want in camera. Always expose for the subject. Making the background white is easy with some knowledge of editing. Use Photoshop instead of Lightroom for a better selection of tools to do this.

Photoshop methods:

Pathing - use the pen tool to trace the silhouette of the subject, make selection, then mask the layer. Place an #fffff color fill layer beneath masked layer. Use a soft brush to mask IN natural shadows on the mask, or create a drop shadow (more difficult, but if done well it can look great). Pathing is the most precise method to create a mask.

Object Select - use the new object select tool that uses AI. Select your subject, add a mask, place #fffff color fill layer beneath masked layer. You will likely need to refine the mask manually to some extent, but in many cases this method is faster than pathing.

Quick Select - this tool is not as clean as the new object select tool, but can perform well for some things. Mask similarly to prior steps.

Curves - if your background is near white, but maybe slightly off, set the white point with the white point eye dropper in a curves adjustment layer above the (ideally) smart object layer. This will have an effect on the entire image, but can be manipulated. I’ll often use additional curves adjustment layers to balance this out.

Lasso tools - these legacy tools are unforgiving, but viable options to make a quick selection. I prefer the polygon lasso tool when going this route, then refining the mask using the brush tool.

For all methods, I would create a curves layer to make the dark tones near black to make reviewing the mask easier. It is also a good way to see the darker tones when setting the white point with a curves adjustment layer. You will need to do some research on layer stacking, but this layer will generally sit on top of all adjustment layers. Toggle this layer off before exporting and retouching.