need help texturing this mesh!

Hi. As I understand, you are converting this mesh from somewhere? It must have already came with uvs.
You should either re-start or comeback to the stage where it was not re-unwrapped by you. In image editing software open this template, import and scale down/move the hair texture so it is in the top-left corner, delete the first layer (the template). Now that your background is transparent, save as png.

In Blender select your hair, switch to Edit mode (TAB), in 3d view select everything with A, in UV editor select everything unless it is already selected, click on Open button next to UVs (if there's none, click on X first to remive current texture), get your newly created texture in the scene, put your UVs there ( G to move, S to scale, R to rotate, following the first two with X or Y will constrain modification to horizontal or vertical axes, following the last one with a number from -359 to 359 will rotate the selection by it)

Hair blend files consist of 3 meshgroups: the original shape and 2 hat chops. When you put on a hat on the sim in CAS, the hair meshgroup switches from the original to the appropriate hat chop to avoid hair clipping into hat. The blend file you import in The Sims 4 Studio should have the same number of meshgroups with corresponding cut numbers as the one you replace.
If you do not want to make your hair compatible with hats, just duplicate your original shape twice ( select it, in 3d view press Shift D, without moving the cursor LMB-click to confirm ) and set different cut numbers.


More about cut numbers here

Your mesh is missing a uv_1 map which is what dictates how it will morph during body customization
Unfold rig, make head_2 selectable ( cursor icon next to eye icon), select your hair, add uv_1 map by clicking on +, add Data transfer modifier. Choose head_2 as Source object, set the rest like in the pic, hit apply;


When transferring weights in newer versions of Blender, you must take an extra step which is to use Clean tool
Append and select the reference mesh, shift-select your hair and transfer weights, use Clean tool;

You need to create it first

- select uv_0;
- switch to Edit mode ( TAB );
- enable sync;
- holding Ctrl to activate lasso selection, hold LMB and circle around these vertices to select all of them;
- Select - Select more/less - more. There's, like, a whole original scalp under the hair that is selected right now;
- in 3d view press Delete and choose Faces;
- there are a few holes in the mesh now. Activate snapping to vertex, select one vertex at a time, press G and snap it to the right one;