There is a belt tensioner tool that has a long handle & attachments to use when there is limited room.You can usually find one at your local parts supplier.
This tool will not hold it for you but give much more leverage to assist you.
Accessory Drive Belts Replacement (VIN K) Removal Procedure
The accessory drive belt controls all engine-mounted accessories. All driven accessories rigidly mount to the engine. A spring-loaded belt tensioner maintains the drive belt tension. A belt squeak, occurring when starting or stopping the engine, it has not effect on belt durability. The drive belt tensioner can control the belt tension over a broad range of belt length. However, there are limits to the tension's compensation ability. When using the tensioner outside of its operating range, poor tension control and/or damage to the tensioner may occur.
Yes,you will need to remove the engine mount.
Leverage is the key to moving the tensioner,use a special tool or a long breaker bar.