The ornamental design for a baywindowcurtainsupport, as shown and described.
FIG. 1 is a perspective environmental view from the front and top of a bay window curtain support showing my new design.
FIG. 2 is front elevation view thereof.
FIG. 3 is rear elevation view thereof.
FIG. 4 is left side elevation view thereof.
FIG. 5 is right side elevation view thereof.
FIG. 6 is top plan view thereof; and,
FIG. 7 is bottom plan view thereof.
The broken line showing of a bay window and portions of the design and support structure is included for the purpose of illustrating environmental structure and forms no part of the claimed design.