Bootloader

Boot Using OPTION key

1. Restart the computer.
2. Immediately after the chime press and hold down the  "OPTION" key.
3. Release the key when the Boot Manager screen appears.
4. Select the disk icon for the system from which you want to boot.
5. Click on the Arrow button under the drive icon.

During usage you can switch boot by using the Startup Disk preference panel on the Mac or the control panel on Windows.

rEFInd might be what you are looking for, but it and any other option will have to deal with SIP (System Integrity Protection) implemented in OSX 10.11+. Keep in mind any third party boot option may be broken by a later macOS update or upgrade. A PRAM reset should get rid of the default boot option and allow you to use the Startup Disk preference to re-select the default boot option. Don't forget about Option Boot.

jmancav wrote:

I already have Mac OSX Mojave and windows 10 on my MacBook pro retina touch bar 2017

I am using bootcamp. I would like to be able to choose upon boot up to choose which OS I would like each time as I like to use each OS at different times for different purposes. Anyone know how to do this? I am not an advanced user so it would have to be relatively easy.

When you use Startup Manager to select a startup disk, your Mac starts up from that disk once, then returns to using the disk selected in Startup Manager. >System Preferences>Startup Disk

How to select a different startup disk - Apple Support