There is no parameter in the Write node to specify output format. Instead, format is indicated by a prefix or an extension when you type the file name. Here is the appropriate syntax:
<prefix>:/<path>/<name>.<frame number variable>.<extension>
The optional <prefix>: can be any valid extension. <path> is the full path name to the directory where you want to render. The <frame number variable> is usually entered as ####, with #### indicating frame numbers padded to four digits.
You can change the padding by substituting #### with any number of hash marks. For example, two-digit padding would be ##, three-digit would be ###, and five-digit would be #####.
With these conventions in mind, suppose you want to save an image sequence called “final_comp_v01” to the TIFF16 format. Here are examples of names that work in the Write node:
tiff16:/<path>/final_comp_v01.####.tiff
/<path>/final_comp_v01.####.tiff16
tiff16:/<path>/final_comp_v01.####
All extensions supported by Nuke may be used for the prefix. See Appendix B: File Formats for a complete list of recognized extensions.
When a prefix is used, it takes precedence over the format represented by an extension. In fact, the prefix makes the extension unnecessary.
You could, for example, enter exr:/<path>/### as the file name, and this would create an OpenEXR image sequence with frame numbers only, padded to three digits.